NBSIR 73-118

Examination of Failed Components From

District of Columbia Incinerator No. 5

T. Robert Shives

Mechanical Properties Section

Metallurgy Division

National Bureau of Standards

Washington, D. C. 20234

March 2, 1973

Failure Analysis Report

Prepared for Construction and Repair Division

Department of Environmental Services

Government of the District of Columbia 20019

NBSIR 73-118

EXAMINATION OF FAILED COMPONENTS FROM

DISTRICT OF COLUMBIA INCINERATOR NO. 5

T. Robert Shives

Mechanical Properties Section

Metallurgy Division

National Bureau of Standards

Washington, D. C. 20234

March 2, 1973

Failure Analysis Report

"This document has been prepared for the use of Construction and Repair Division, Department of Environmental Services, Government of the District of Columbia. Responsibility for its further use rests with

that agency. NBS requests that if release to the general

public is contemplated, such action be taken only after consultation with the Office of Public Affairs at the National Bureau of Standards."

U. S. DEPARTMENT OF COMMERCE, Frederick B. Dent, Secretary

NATIONAL BUREAU OF STANDARDS, Richard W. Roberts, Director

TABLE OF CONTENTS

Page

SUMMARY vi

1 . INTRODUCTION 1

1.1 Reference 1

1.2 Parts Submitted 1

1.3 Background Information 2

2. PURPOSE 2

3. EXAMINATIONS, TESTS, AND ANALYSES PERFORMED 3

3.1 Visual Examination 3

3.2 Tensile Tests 3

3.3 Metallographic Examination 3

3.4 Chemical Analyses 4

3.5 Hardness Measurements 4

3.6 Fractographic Examination 4

4. RESULTS OF EXAMINATIONS, TESTS, AND ANALYSES 5

4.1 Visual Examination 5

4.1.1 Grates 5

4.1.2 Trunnions , 5

4.1.3 Trunnion pins 6

4.1.4 Trunnion sockets 6

4.1.5 "Spacers" 6

4.1.6 Residue ash conveyor system chain links .. 7

i

Page

4.2 Tensile Tests 7

4.2.1 Grates 7

4.2.2 Chain links 8

4.3 Metallographic Examination 8

4.3.1 Grates 8

4.3.2 Trunnions 9

4.3.3 Trunnion pins 9

4.3.4 Trunnion sockets 9

4.3.5 Chain links 10

4.4 Chemical Analyses 10

4.4.1 Grates 10

4.4.2 Trunnions 11

4.4.3 Trunnion pins 11

4.4.4 Chain links 11

4.5 Hardness Measurements , 11

4.5.1 Grates 11

4.5.2 Trunnions 12

4.5.3 Trunnion pins 12

4.5.4 Trunnion sockets 12

4.5.5 Chain links 12

4.6 Fractographic Examination 12

4.6.1 Trunnions 12

4.6.2 Trunnion pins 13

4.6.3 Chain links 13

ii

Page

5. DISCUSSION OF RESULTS 14

5.1 Grates 14

5.2 Trunnions 15

5.3 Trunnion Pins 15

5.4 Trunnion Sockets 16

5.5 "Spacers" 17

5.6 Chain Links 17

5.7 General Discussion 17

5.7.1 Incinerator components except chain links.. 17

5.7.2 Chain links 18

6. CONCLUSIONS 18

6.1 Incinerator Components Except Chain Links 18

6.2 Chain Links 19

7. ACKNOWLEDGEMENT 2 0

REFERENCES 2 0

TABLES

1. RESULTS OF TENSILE TESTS 21

2. RESULTS OF CHEMICAL ANALYSES 2 2

3. RESULTS OF HARDNESS MEASUREMENTS 2 3

FIGURES

1. Bottom view of grates as received 24

2. New trunnion, trunnion pin, and trunnion socket as

received at left. Used trunnion, trunnion pin,

and trunnion socket as received at right 25

3. New "spacer" at right, used "spacer" at top left,

and used, fractured trunnion at bottom left as

received 2G

iii

Page

FIGURES CContinued)

4. Two used, fractured chain links at left and center,

and one used, intact chain link attached to a two-piece connecting unit as received 27

5. Part of the bottom "grille" work of the used grate

showing attached material and distortion 2 8

6. Unetched microstructure of cross section through

top of the used grate 2 9

7. Etched microstructure of cross section through the

bottom of the new grate 2 9

8 . Unetched microstructure of cross section through

used grate bottom showing graphite nodules and

secondary graphite 3 0

9. Etched microstructure of cross section through

used grate bottom showing graphite nodules,

some of which are surrounded by secondary graphite. 3 0

10. Etched microstructure of cross section through

globule no. 2 and the adjacent portion of the

grate bottom 31

11. Unetched microstructure of cross section through

the new trunnion 31

12. Unetched microstructure of cross section through

the used trunnion 32

13. Etched microstructure of cross section through

the new trunnion 3 2

14. Etched microstructure of cross section through

the used trunnion 3 3

15. Unetched microstructure of cross section through

new trunnion pin 3 3

16. Etched microstructure of cross section through

new trunnion pin 3 4

17. Etched microstructure of cross section through

used trunnion pin 3 4

iv

Page

FIGURES (Continued)

18. Unetched microstructure of cross section through

the new trunnion socket 35

19. Etched microstructure of cross section through

the new trunnion socket 3 5

20. Etched microstructure of cross section through

used trunnion socket 3 6

21. Unetched microstructure of cross section through

chain link 3 6

22. Etched microstructure of cross section through

chain link 37

23. Fracture surface of one of the failed trunnions 37

24. Fracture surface of one of the failed chain links ... 3 8

V

SUMNARY

The Department of Environmental Services of the

Government of the District of Columbia submitted a number

of components from Incinerator Number Five to the NBS

Mechanical Properties Section for examination. These included

grates, trunnions, trunnion pins, trunnion sockets, "spacers",

and chain links from the residue ash conveyor system.

According to the Department of Environmental Services, many

of the incinerator components were failing within a relatively

short time after installation.

Examination of both new and used components (excluding

the chain links) revealed that all of the used components,

had undergone significant mechanical property and microstructural changes when compared to the material of the new

components. Some of the used components also exhibited some

distortion and loss of material. The changes in the used

components could be attributed to exposure of the materials

to temperatures which were excessive for the materials employed.

The new and used grate material did not meet contract

specifications for tensile properties and hardness. Where

specifications were available, the other components did meet

mechanical property specifications.

The materials met chemical composition specifications

where specifications were given.

Fractures in the trunnions and the trunnion pins

appeared to have been due to tensile overload, and the

fractures in the chain links appeared to have been due to

impact or tensile overload.

vi

Examination of Failed Components from

District of Columbia Incinerator No. 5

1. INTRODUCTION

1 . 1 Reference

Government of the District of Columbia, Department of

Environmental Services, Construction and Repair Division.

Request by Mr. Edward M. Halley, Jr., Chief, Bureau of

Construction and Repair. Job. No. 206-001.

1.2 Parts Submitted -

The D.C. Department of Environmental Services submitted

a number of parts from Incinerator No. 5 for examination.

These parts are listed below and are shown as received in

figures 1 through 4.

1. One new grate with both trunnions and trunnion

pins in place (figure la)

.

2. One used grate with one trunnion and the mating

trunnion pin in place (figure lb)

.

3. One new trunnion and trunnion pin (figure 2) .

4, One used, fractured trunnion and trunnion

assembly (figure 2)

.

pin

5. One used, fractured trunnion (figure 3)

.

6. One new trunnion socket (figure 2)

.

7. One used trunnion socket (figure 2)

.

8. One new "spacer" (figure 3)

.

9. One used "spacer" (figure 3)

.

10. Two fractured chain links from the residue ash

conveyor system (figure 4)

.

11. One used, intact chain link attached to one twopiece connecting unit from the residue ash

conveyor system (figure 4)

.

- 1

-

In addition, a copy of the Government of the District

of Columbia, Department of Sanitary Engineering Bid Proposal

and Specifications for Invitation No. C-7038-S for

Incinerator No. 5 was furnished. Any reference made hereafter to "specification" or "specifications" not specifically

identified refers to this document.

1 . 3 Background Information

Information in this section was furnished by Mr. Alan

Cassel of the D. C. Department of Environmental Services.

Incinerator No. 5, which is of recent construction, contains

a number of grates (such as that shown in figure la) on which

trash is burned. The grates are "hinged" at the trunnions

and are raised at regular intervals moving the burning trash

from one grate to another. Combustion is promoted by the

blowing of air through the grates from below. This air

stream is also supposed to cool the grates to keep them from

becoming overheated. The rate of air flow should control

the rate of combustion and the temperature of the grates.

Grates, trunnions, trunnion pins, and trunnion sockets have

been failing in a relatively short time, sometimes within

two to three months of the time they are put into service.

A number of links of the conveyor chain from the residue

ash conveyor system had fractured, again in a relatively short

time. This system is used to remove ash remaining after the

trash is burned from the incinerator for disposal. According

to the information furnished to us, the chain is not subjected

to hot ash and its temperature never reaches that of boiling

water (212° F)

.

2. PURPOSE

The D. C. Department of Environmental Services requested

the NBS Mechanical Properties Section to conduct a failure

analysis of the submitted parts as follows:

1. Tensile tests (ultimate tensile and yield

strengths and elongation) on specimens from the

new grate, the used grate, and one of the

fractured chain links.

2. Hardness measurements on material from both grates,

the trunnions, the trunnion pins, and the chain

links.

- 2 -

3. Chemical composition determinations for material

in one grate, one trunnion, one trunnion pin,

and one chain link.

4. Metallographic examination of material from both

grates, one trunnion, one trunnion pin, and one

chain link.

5. Fractographic examination of one of the fractured

trunnions, one of the trunnion pins, and one of

the chain links.

In addition, if the material currently used for the

grates appears to be unsatisfactory, recommendations of

better materials were requested.

3 . EXAMINATIONS, TESTS, AND ANALYSES PERFORMED

3.1 Visual Examination

The various components were examined visually for

apparent damage. A limited number of dimensional comparisons

were made between new and used components

.

3 . 2 Tensile Tests

Two tensile specimens each were machined from the used

grate, the new grate, and one of the fractured chain links

to sizes and types in accordance with ASTM Standard E8-69.

One specimen was taken from each grate bottom with the

longitudinal axis of the specimen parallel to the bottom of

the grate. The other grate specimens were taken near the

top in the same vertical plane as the specimens raken at the

bottom. Specimens from the bottoms of the grates were

designated Ul and Nl for the used and new grates, respectively.

Specimens from the tops of the grates were designated U2 and

N2 for the used and new grates, respectively. Chain link

specimens were designated Cl and C2 . The tensile tests were

run at cross head speeds of 0.01 to 0.016 inch per minute.

3 . 3 Metallographic Examination

Metallographic specimens were prepared from cross sections

through the top and bottom of both the new and used grates,

new and used trunnions, new and used trunnion pins, new and

Used trunnion sockets, and one of the fractured chain links.

- 3, -

It was deemed advisable to examine metallographically more

specimens than had been originally proposed because of the

results of the hardness measurements which indicated that

there were differences in the mechanical properties between

essentially all of the new parts and the corresponding used

parts. Cross sections for examination from the used grate

bottom were taken through attached "globules" so that the

material in these globules could be examined as well as the

grate material.

3 . 4 Chemical Analyses

One sample each was selected from the new grate bottom,

a new trunnion, a new trunnion pin, and one of the fractured

chain links for chemical analysis. Surface scale, paint,

and the apparent decarburized layer (on the chain link sample)

were removed from the surfaces of the sample before they were

submitted to a commercial laboratory for chemical analysis.

Analyses were made using spectrographic techniques.

In addition, qualitative analyses were made on cross

sections through the tv/o globules from the bottom of the used

grate on which the hardness measurements were made. This was

carried out with the non-dispersive X-ray analyzer used in

conjunction with the scanning electron microscope.

3 . 5 Hardness Measurements

Brinell hardness measurements (BHN) were made on the

metallographic specimens. Knoop microhardness measurements

(KHN) were made at 200 grams load on two cross sections

through "globules" of material attached to the bottom of the

used grate. Knoop microhardness measurements were also made

on a cross section through one of the fractured chain links

near the surface because of the presence of what appeared to

be a decarburized layer at the surface.

3 . 6 Fractographic Examination

The fracture surfaces of the two failed chain links, the

failed trunnions and the failed trunnion pins were examined

visually and macroscopically in an attempt to determine the

mode(s) of fracture.

- 4 -

4. RESULTS OF EXAMINATIONS, TESTS, AND ANALYSES

4 . 1 Visual Examination

4.1.1 Grates

The used grate was covered with a fairly heavy

scale at and near the bottom and on most of the inside

surface. The new grate had an apparently superficial layer

of rust on m.ost of the surface except for the area which had

been painted. There was considerable distortion in the

bottom "grille" work and along both sides of the used grate

(figures lb and 5) as compared with the new grate (figure la)

.

There were various bits of material attached to the grille of

the used grate (figure 5) . Some of this material could be

easily removed, but most of it was firmly attached to the

grate, and some of the material was in globular form appearing

as though it had melted and resolidified on the grate bottom.

The widths of the dividing members of the grille work

measured 0.7 5 inch for the new grate compared to 0.5 0 inch

for the used grate. These values are an average of measurements made on 15 dividing members of each grate. The widths

of the side panels of both the new and the used grates

measured essentially the sam.e , about 3/4 inch.

One of the trunnion pins from the used grate had

apparently fractured, and it and the com.panion trunnion were

separated from the grate.

4.1.2 Trunnions

In addition to the three trunnions attached to

the grates (figures la and lb) , there were two used and one

new trunnion submitted for examination (figures 2 and 3)

.

The outer surfaces of the two new trunnions attached to the

new grate were painted. The new trunnion not attached to a grate was not painted and had a smtall am.ount of v/hat

appeared to be superficial rust on the surface, but otherwise

appeared to be similar to the attached trunnions.

The one trunnion attached to the used grate was covered

with scale, and there was a light grey deposit adjacent to

the head of the trunnion pin. This trunnion-trunnion pin

assembly appeared to be bent relative to the grate. The other

trunnion-trunnion pin assembly for this grate was missing,

and apparently at least the pin had fractured. Some of the

- 5 -

light grey deposit was also located in the cavity in the used

grate where the assembly had been. Both of the used unattached

trunnions had fractured. One of them exhibited very little

rust and there was still considerable bright metal showing.

The appearance of this trunnion compared to the appearance of

the other used trunnions indicated that this trunnion may

have been in use for a relatively short period of time, or

was in an area of the incinerator which was not subjected to

the heat of the burning trash. Both the other unattached used

trunnion and the attached used trunnion were covered with a

fairly heavy scale.

The outside diameters of all the trunnions except for

the used one attached to the used grate were essentially the

same and measured about 2.95 inches. The diameter of the

used trunnion attached to the used grate measured 2.9 0 inches.

4.1.3 Trunnion pins

Of the two trunnion pins attached to the new grate,

only the heads were visible, and these were painted. The new

unattached trunnion pin was not painted and had a small amount

of superficial rust. The shaft of the pin was straight. The

heads of the two used trunnion pins were covered with scale.

The unattached pin had apparently fractured (figure 2) close

to the point of attachment to the grate, and the shaft was

bent. The shaft of the attached trunnion pin also appeared

to be bent.

4.1.4 Trunnion sockets

One new and one used trunnion- socket v/ere submitted

for examination (figure 2) . The used socket was covered with

a heavy scale. The new socket was painted only in places and

was covered with a superficial rust where there was no paint.

The diameter of the hole of the new socket measured 3 . 0 inches

and that of the used socket measured 3.1 inches. The widths

of the tops of the sockets (measured parallel to the center

line of the hole) were 0.99 inch for the new one and 0.67

inch for the used one. There was much less difference in the

widths at the bottom where the measurements were 0.9 8 inch

for the new socket and 0.97 inch for the used socket.

4.1.5 "Spacers"

The used "spacer" was covered with a heavy scale

and exhibited considerable distortion. The new one was partially covered with paint, and the unpainted areas were covered

with a superficial rust. Enough material had been lost from

one area of the used "spacer" so that the side of the "spacer"

intersected one of the two bolt holes. This can be seen in

figure 3.

- 6 -

4.1.6 Residue ash conveyor system chain links

Both of the fractured chain links had patches of

a grey, powdery substance on their surfaces. In the areas

not covered with this substance, the link surfaces were

covered with scale. One of the fractured links had separated

into at least two pieces, and only the piece shown in figure

4 (link in the center of the photograph) was submitted. One

of the two fracture surfaces shown in the figure was about

75% covered with what appeared to be the same grey substance

found on the link surfaces. The intact link was covered with

scale and there was no evidence of the grey substance visible

to the eye. There was no apparent distortion in any of the

links, but in the fractured link which had not separated, the

fracture faces had separated slightly.

4 . 2 Tensile Tests

4.2.1 Grates

The results from the tensile tests of material from

the grates are presented in the first four rows of Table 1.

The material in the grates was specified to have the following

tensile properties:

Ultimate tensile strength 100,000 psi minimum

Yield strength 70,000 psi minimum

Elongation 3 % minimum

Of the four specimens tested, only the one from the new grate

bottom had an ultimate tensile strength above the specified

minimum, although the specimens from the new grate top and

the used grate top had ultimate tensile strengths within

4 1/2% of the specified minimum. The ultimate tensile strength

of the specimen from the bottom of the used grate was quite low.

Only the specimen from the top of the new grate had a yield strength (0.2% offset) above the specified minimum,

although the specimens from the bottom of the new grate and

the top of the used grate had yield strengths within about

3 1/2% of the specified minimum. The yield strength for the

material from the bottom of the used grate was quite low.

The elongation (in one inch) was above the specified

minimum for all specimens except the one from the top of the

new grate. However, elongation for the one from the top

of the used grate was very high.

- 7 -

4.2.2 Chain links

The results from the tensile tests of material

from the chain link are presented in the last two rows of

Table 1. The material in the chain links was specified to

have the following tensile properties:

Ultimate tensile strength 230,000 psi

Breaking strength 228,000 psi

Yield point 210,000 psi

The yield strengths and the ultimate tensile strengths for

both chain link specimens were well above the specified

values.

4 . 3 Metallographic Examination

4.3.1 Grates

The microstructure of cross sections taken from

both the top and the bottom of the new grate and the top of

the used grate were essentially the same. A typical unetched

microstructure representative of these areas is shown in

figure 6. The cross section shown was taken from the top of

the used grate. The dark, round areas are graphite nodules.

The material appears to be relatively free of inclusions and

no porosity is evident. An etched microstructure representative of this material (figure 7, cross section through bottom

of new grate) shows the graphite nodules in a matrix consisting

of essentially all pearlite. This appears to be an as-cast

structure

.

A different microstructure is exhibited by the material

from the bottom of the used grate. An unetched microstructure

of a cross section through the bottom of the used grate is

shown in figure 8 (at a higher magnification than figures 6

and 7) . There is secondary graphite surrounding some of the

graphite nodules. An etched microstructure from this cross

section (figure 9) shows that the matrix is mostly ferrite

with a small amount of spheroidized cementite instead of

pearlite. The presence of these constituents indicates that

the material was probably heated to temperatures in the range

of 1200 to 1300** F for an appreciable period of time.

An etched microstructure of a cross section through one

of the globules (no. 2 in Table 3) where it was attached to

the grate bottom is shown in figure 10. The material appears

to have fused to the grate bottom.

- 8 -

4.3.2 Trunnions

The unetched microstructures of cross sections

from the new and the used trunnions are shown in figures 11

and 12, respectively. The dark areas appear to be graphite,

or, in some places, cavities where graphite was pulled out

during metallographic preparation. These figures may not

represent typical concentrations of the graphite, but they

do illustrate the relative sizes and shapes of the particles.

Etched microstructures of cross sections through the new

and used trunnions are shown in figures 13 and 14, respectively.

A dendritic structure of prior austenite is exhibited in both

cases. In figure 13, the microstructure consists of cementite

(white area) , martensite (grey area) , and what appears to be

unresolved pearlite (dark area, could not be resolved up to

X 800) . The microstructure of the used trunnion (figure 14)

has essentially an as-cast appearance and consists of cementite

and tempered martensite.

4.3.3 Trunnion pins

The unetched microstructure of a cross section

through the new trunnion pin is shown in figure 15. This is

typical for the material in both the new and the used

trunnion pins. The steel is very clean and has very few

inclusions

.

An etched microstructure of a cross section through the

new trunnion pin is shown in figure 16. It appears to be a

normalized structure consisting of pearlite in a pro-eutectoid

ferrite network.

The microstructure of the used trunnion pin (figure 17)

consists of spheroidized carbides (probably mostly iron carbide,

Fe3C) in a ferrite matrix. The spheroidized microstructure

can be formed from the norm.alized structure by holding the

material at an elevated temperature for a long period of time.

4.3.4 Trunnion sockets

The trunnion sockets were specified to be

manufactured from "wear resistant alloyed iron"l. Material

from both the new and the used trunnion sockets appear similar

in the unetched condition. A typical unetched microstructure

of a cross section through the new trunnion socket is shown

in figure 18. The dark, round particles are graphite nodules.

- 9 -

The etched microstructure of a cross section through the

new trunnion socket is shown in figure 19. The microstructure

consists of graphite nodules in a matrix containing primarily

pearlite (grey) with some massive cementite (white)

.

The etched microstructure of a cross section through the

used trunnion socket is shown in figure 20. The microstructure

here consists of graphite nodules in a matrix of ferrite (white)

,

spheroidized carbides (appearing as small dark dots) , and a very small amount of pearlite.

4.3.5 Chain links

An unetched microstructure of a cross section taken

through a used chain link and intersecting the surface of the

link is shown in figure 21. There appears to be some corrosion

or scale that has penetrated the surface. A few inclusions

can be seen, but the material appears to be relatively clean.

An etched microstructure of a cross section intersecting

the surface of the link is shown in figure 22. The microstructure consists primarily of very fine needles of tempered

martensite. Near the surface, however, there are areas which

appear to have been decarburized . A typical area of decarburization is indicated by arrow A. The phase appearing white

in the photomicrograph is ferrite. The depth of the decarburized layer is not uniform. At the surface on the right of

figure 22 (arrow B) , the microstructure is predominantly

martensite. Some of the surface may have been removed by

grinding after the links were fabricated or heat treated,

or by wear o^ corrosion in service.

4 . 4 Chemical Analyses

The results of the chemical analyses are presented in

Table 2.

4.4.1 Grates

The grate material was specified to be "high quality,

heat resistant ductile iron"-^. The results of the chemical

analysis indicate that the material of the new grate is

ductile iron^

.

A qualitative analysis of the material in two globules

(designated globules no. 1 and 2 in Table 2) indicates that

globule no. 1 consists basically of iron with some aluminum,

copper, and silicon present, and that globule no. 2 is nearly

all iron with a small amount of aluminum.

- 10 -

4.4.2 Trunnions

The trunnions were specified to be manufactured

from "wear resistant alloyed iron"-^. The chemical analysis

indicates that the material of the new trunnion is a nickelchromium iron which apparently has no commercial designation.

4.4.3 Trunnion pins

No chemical composition specification was available

for the trunnion pin material. Analysis indicates that the

material of the new trunnion pin is a plain carbon AISI 1050

steel,

4.4.4 Chain links

The specified material for the chain links is

"heat treated alloy steel"-^. The chemical analysis indicates

that the material is similar to AISI S5 tool steel.

4 . 5 Hardness Measurements

The results of the hardness measurements are presented

in Table 3. When the measured value is a Knoop number (KHN)

,

a Brinell hardness number (BHN) approximately equivalent to

the measured value is given in the last column for comparison.

Each tabulated number represents an average of at least three

measurements.

4.5.1 Grates

According to specifications''", the Brinell hardness

of the grate material should be approximately BHN 28 6. There

are no significant differences among the hardness values for

specimens from both the new grate top and bottom and the used

grate top. All of the results are, however, lower than the

approximate BHN specified. The specimen from the bottom of

the used grate had a hardness considerably lower than that of

the other specimens and more than 115 Brinell hardness points

below the specified approximate value.

Hardness measurements taken on two globules of material

attached to the bottom of the used grate indicate that one

was harder than any of the grate material examined, and the

second was slightly softer than the adjacent material in the

bottom of the used grate.

- 11 ~

4.5.2 Trunnions

The material in the trunnions should have an approximate BHN of 300l. The hardness values obtained for both

new and used trunnions were well above this number. The nev/

trunnion was somewhat harder than the used one

.

4.5.3 Trunnion pins

There was no hardness specification available

for the trunnion pins. The used trunnion pin was considerably

softer than the nev; one.

4.5.4 Trunnion sockets

No hardness specification was available for the

trunnion socket material. The new trunnion socket was

significantly harder than the used one, and the trunnion

sockets were significantly softer than the trunnions.

4.5.5 Chain links

Hardness measurements made on a cross section

through one of the fractured chain links indicated that the

hardness of the material away from the edge (surface of the

link) was within the specified range of BHN 4 60 to 512-'-.

There was a significant decrease in hardness at the edge which

was apparently due to decarburization (see section 4.3.5) .

4 . 6 Fractographic Examination

4.6.1 Trunnions

The fracture surfaces of the fractured trunnions

were in poor condition for examination when submitted . They

were corroded and showed evidence of mechanical damage . One

of the better preserved fracture surfaces is shown in figure

23. The fracture surface has a rough, coarse-featured

appearance. There was no evidence found to indicate fatigue

or any other relatively slowly propagating crack mechanism

contributing to the failure. Fracture was likely due to

overload, perhaps caused (at least in part) by a stress

concentration brought about by the bending of the softened

trunnion pin.

- 12 -

4.6.2 Trunnion pins

The fracture surface of the failed trunnion pin

was heavily coated with scale. There appeared to be some

reduction in cross-sectional area of the pin adjacent to the

fracture, indicating that the fracture may have been caused

by a tensile overload.

4.6.3 Chain links

One of the chain link fracture surfaces was largely

covered with a grey material which could be easily crumbled

and removed from the surface. This material can be seen as

the light area on the fracture surface at the bottom center in

figure 4. The material looked and felt somewhat like ash.

That part of the fracture surface not covered with this

substance was covered with what appeared to be a heavy rust.

Removal of some of the grey substance revealed corrosion

(rust) on that part of the fracture surface covered with the

substance as well.

The fracture surface was fairly flat with coarse features.

The fracture had occurred in a brittle manner and probably

from either impact or overload. If the grey substance on the

fracture surface was indeed ash, this chain link was apparently

kept in service after the side containing this fracture had

failed

.

The second chain link fracture surface examined is shown

in figure 24. The part of the fracture surface adjacent to

the outside surface of the chain link was fibrous in nature

(upper part of fracture surface shown in figure 24a) . Part

way through, the fracture surface makes a sharp change in

direction (arrows in figure 24b) , and the part of the fracture

surface adjacent to the inside surface of the chain link is

flat and smoother than the fibrous part. There appears to

have been a pre-existing crack (dark part of fracture surface

indicated by arrow A, figure 24a) . This may have been a quenching crack which formed during heat treatment of the link.

It appears that the fracture probably initiated at the outside

surface of the link (arrow B, figure 24a) , and propagated

inward, passing through the pre-existing crack. The fracture

in this chain link probably occurred from either impact or

overload

.

- 13 -

5. DISCUSSION OF RESULTS

5 . 1 Grates

The results of the chemical analysis and the metallographic examination indicated that the material from the

new grate was ductile (nodular) iron. In no case was the

measured hardness of the material from either grate as high

as that specified, and material from the new grate failed to

meet tensile property specifications, although not by a wide

margin.

Assuming that the used grate was essentially the same

before service as the new grate, the mechanical properties,

physical dimensions, and the microstructure of the material

from the bottom of the used grate underwent significant

changes while in service. The material from the top of the

used grate did not undergo these changes and was similar to

that of the new grate.

The microstructure of the material from the bottom of

the used grate appears similar to that expected from an

extended annealing heat treatment between about 12 0 0 and

135 0° F. The extent of the decomposition of the cementite

indicates that the material was held at a temperature of

approximately 1300° F for many hours total. The decrease in

hardness and tensile properties, and the changes in dimension and microstructure of the material from the bottom of

the used grate could all have been caused by the grate bottom

being subjected to excessive temperatures for this material.

The distortion in the bottom members could have been produced

by thermal cycling to temperatures near and/or above the

eutectoid temperature (about 133 0° F)

.

Since the bottom of the grate had undergone changes due

to exposure to heat that the top of the grate did not suffer,

the grate was apparently subjected to a temperature gradient.

The temperature at the grate top was never great enough to

have produced any significant effect on the material properties,

while the grate bottom was subjected, probably repeatedly,

to relatively high temperatures.

The fact that the material from the new grate does not

quite meet the mechanical property specification does not

appear to be a problem in itself, as the properties are not

below the specified minimum values by a large amount. There

is reason to believe that even if the material did meet

specifications before use, it would still have mechanical

properties after use essentially the same as those of the

used grate examined, and therefore would likely fail if

subjected to similar conditions.

- 14 -

5.2 Trunnions

Chemical analysis of material from a new trunnion indicated

that the trunnions were fabricated from a nickel-chromium cast

iron which is an alloy cast iron.

There was only a slight difference in hardness between

the new and used trunnions, the used one being softer. At

this high hardness, little ductility would be expected.

There were, however, significant differences between the

microstructures of the new and used trunnions. The presence

of tempered martensite and the lack of pearlite in the used

trunnion indicates that this trunnion was subjected to temperatures greater than 1330° F, and then cooled rapidly. This

change in microstructure is consistant with the changes in

the microstructure of the used grate bottom. In both cases,

the changes could have been brought about by excessive

temperatures

.

The fracture surfaces of the failed trunnions were

corroded and had suffered "damage" which obscured many of the

fracture features. The apparent "damage" may have been

mechanical in nature, but it may also have been produced by

temperature excursions. Failure appears to have occurred due

to overload. The trunnion-trunnion pin assembly still

attached to the used grate was bent, as was the used pin which

was not attached. If the trunnion pin bending is a common

occurrence, stress concentrations could be imposed on the low

ductility trunnion causing overload failure. High loads at

about 1330° F during heating or cooling could have caused

shear fracture with very little ductility in this material.

5 . 3 Trunnion Pins

Chemical analysis indicated that the material in the new

trunnion pin V7as probably a plain carbon AISI 1050 steel.

Hardness measurements indicated that the unattached, used

trunnion pin was considerably softer than the new one, and

the new trunnion pin was much softer than the trunnions.

Assuming that the used trunnion pins before service were

essentially the same as the new trunnion pin, a significant

change in microstructure took place in the used trunnion pin.

Both the decrease in hardness and the changes in microstructure could have been caused by subjecting the trunnion

pins to temperatures considered excessive for this material.

- 15 -

t

The conversion of the normalized structure of the new

pin to the spheroidized carbides in ferrite structure of the

used pin would require temperatures in the range of about

1300 to 1330° F for an appreciable time. This could have

occurred through a series of temperature excursions and is

consistent with the changes found in the used grate bottom

and the used trunnion.

The unattached used pin, discussed above in regard to

hardness and microstructure , was bent v/hen submitted, and the

used trunnion pin attached to the used grate appeared to be

bent also. This indicates that the trunnion pins apparently

do not have the tensile strength required to function

properly, at least at the temperatures to which they were

subjected

.

5 . 4 Trunnion Sockets

If the used trunnion socket was essentially the same

before use as the new trunnion socket, there was a considerable

loss of material at the top of the used socket, and there was

a significant decrease in the hardness of the used socket.

The hardness of the new trunnion socket was considerably less

than that of the trunnions (a trunnion being the mating part

to the socket) . Both of these changes can be attributed to

the trunnion sockets being subjected to temperatures

considered excessive for this material.

The significant change in microstructure from graphite

nodules in a matrix consisting primarily of pearlite with some

massive cementite in the new trunnion socket to mostly

spheroidized carbides in a ferrite matrix could have been

brought about by exposure to temperatures of from 12 0 0 to

1300° F for an appreciable length of time, or by a number of

temperature excursions into this temperature range.

The changes in the used trunnion socket material are

consistent with the changes found in the materials from the

other components discussed previously.

A considerable amount of a grey, powdery-appearing

substance was found in the used trunnions and in the cavity in

the used grate where a trunnion-trunnion pin assembly had been.

This substance appears to be ash residue. None of this was

found in the used socket, but the proximity of the trunnions

and the trunnion sockets may make it likely that this substance

could get into the sockets with relative ease. If this

substance has an abrasive nature, it could cause excessive

wear in the relatively softer sockets.

- 16 -

5 . 5 "Spacers"

The "spacers" were subjected to a visual examination only.

There had been a considerable loss of material in the used

"spacer" compared to the new one. This could have occurred,

as in the other components, because of excessive operating

temperatures for the material from which the "spacers" were

fabricated

.

5.6 Chain Links

The chain link material meets the specifications for

chemical composition, tensile properties, and hardness

(except for the decarburized surface layer)

.

The fractures in the two links examined appeared to be

the result of impact or overload, and one of the fractures

passed through a pre-existing crack. This pre-existing crack

is considered very detrimental in that it reduces the effective

cross sectional area of the link and it acts as a staff

concentrator. The material was quite hard, and as a result,

would not be expected to have much ductility or toughness.

The decarburized surface layer indicates an improper

atmosphere during heat treatment. This layer is considered

detrimental in that it results in the outer layer of the

links having a relatively low hardness and thus low wear

resistance.

5 . 7 General Discussion

5.7.1 Incinerator components except chain links

The apparent principal problem leading to the

failure of the incinerator components has been the exposure

of these components to excessive temperatures for the

materials from which they were fabricated. This indicates

that the operating temperatures were too high for the

materials in the area of the incinerator where they were

exposed. There are basically two approaches to the solution

of the problem: 1) lower the operating temperatures in these

areas sufficiently so that acceptable lifetimes for the

various components can be attained, or, alternatively, 2)

specify materials which will withstand the higher operating

temperatures

.

- 17 -

The first approach would be the easiest and also the

least expensive to implement. According to information

supplied to us (section 1.3), the temperature can be

controlled, at least in part, by regulating the rate of the

air blast through the grates, or making it more uniformly

distributed. This solution would be feasible, of course,

only if the incinerator will operate properly at the reduced

temperature

,

The second approach would be more expensive in that all

components of the types examined by NBS would have to be replaced with components fabricated from materials capable of

withstanding the higher temperatures. In addition to the

cost of a complete change of components , the new high

temperature materials would likely be much more costly than

those currently in use.

Utilizing the first approach, there is a possibility that,

even with operating temperatures low enough so that the

mechanical properties of the components are unaffected by

temperature, the trunnion pins may have insufficient strength

to function properly. If this was found to be the case, a stronger material should be used. One way to accomplish this

would be to heat treat the material currently in use to a higher strength. If the trunnion pins function properly, the

probability of trunnion failure should be reduced.

5.7.2 Chain links

A decrease in the probability of chain link

failure could likely be attained by substituting case-hardened

components for the currently used through-hardened components

.

A case-hardened chain link (with no decarburized surface

layer) would provide a hard surface layer for good wear

resistance. A core softer than that used currently would

provide a greater degree of ductility and toughness.

6. CONCLUSIONS

6 . 1 Incinerator Components Except Chain Links

1. With the exception of the top of the used grate and

the possible exception of the used trunnion, all

submitted used components examined had suffered

considerable impairment of mechanical properties,

apparently due to exposure to excessive temperatures

for the material.

- 18 -

2. The used grate, used "spacer", and used trunnion

socket had suffered significant losses of material,

also probably due to exposure to excessive temperatures for the material.

3. The used grate and the used "spacer" exhibited

considerable distortion, probably caused by exposure

to excessive temperatures for the material.

4 . Material from both the new and used grates did not

meet tensile property or "approximate" hardness

requirements given in the contract specifications.

5. Material from both the new and used trunnions met

hardness specifications. (No hardness specifications

were available for components other than the grates

and the trunnions .

)

6. One, and probably a second, trunnion pin was bent

indicating that the trunnion pins may not have

sufficient strength to function properly at the

temperatures to which they were subjected.

7. Material from all used components examined, had

undergone significant changes in microstructure

(compared to the microstructures of the new

components) which can be attributed to exposure

to excessive temperatures for the material.

8. The chemical analyses of materials from the new grate

. and the new trunnion indicated that the materials

were ductile cast iron and alloyed cast iron,

respectively. (No specification was available for

the trunnion pin chemical composition.)

9. The fractured trunnions appear to have failed from

overload, perhaps from a stress concentration

imposed by the bending of the trunnion pins

.

10. The used fractured trunnion pin appeared to have

failed due to tensile overload.

6 . 2 Chain Links

1. Material from the chain link examined met contract

specifications for chemical composition and

mechanical properties^.

2. The fractured links appear to have failed from

impact or tensile overload.

- 19 -

3. There was a detrimental decarburized layer of nonuniform depth on the surface of the link cross

sectioned which decreased wear resistance.

4. There was a grey, powdery substance covering parts

of the surfaces of both fractured chain links, and

a large portion of one of the fracture surfaces

was covered with what appeared to be the same

substance

.

7. ACKNOWLEDGEMENT

Mr. L. C. Smith of the Mechanical Properties Section

performed the metallographic specimen preparation and

assisted in some of the tests and examinations.

REFERENCES

1. Government of the District of Columbia, Department of

Sanitary Engineering, Bid Proposal and Specifications

for Invitation No. C-7 038-S, Incinerator Number Five,

1969.

2. Metals Handbook, American Society for Metals, Eighth

Edition, Volume 1, 19 61.

TABLE 1. RESULTS OF TENSILE TESTS

Specimen no. and

identification

Nl Bottom new grate 113 ,600 67 , 500 4 . 6

N2 Top new grate 95,600 7 2,000 1

.

8

Ul Bottom used grate 70,000 49,500 13 . 7

U2 Top used grate 98 ,600 68,500 3 . 0

CI Used chain 288,300 262,4 00 8 . 9

C2 Used chain 284 ,300 254,500 10. 4

1 Values given to the nearest 100 psi

2 Values given to the nearest 0.1 Q.&

Ultimate 0.2% offset % elongation

tensile yield in one inch^

strength,-^ strength,

psi psi

- 21 -

TABLE 2. RESULTS OF CHEMICAL ANALYSES

Quantitative Analyses

Element Specimen

New grate New New Used

bottom trunnion trunnion chain

pin link

Carbon (total)

Carbon (graphitic)

Manganese

Phosphorus

Sulfur

Silicon

Nickel

Chromium

Molybdenum

Copper

3.57 %

2.54

0.46

0.039

0.006

2.26

2.96 %

0.24

0.015

0. 024

1.28

3 .85

2.20

< 0.05

< 0. 05

0.50 %

0.78

0.009

0.015

0.19

< 0. 05

< 0. 05

< 0 . 05

< 0. 05

Element Detected

Qualitative Analyses

Level of Detection

Globule No. 1 Globule No

0.63 %

0.73

0. 015

0. 016

1.58

0.11

0.29

0. 27

0.16

Iron

Aluminum

Copper

Silicon

very strong

weak

very weak

very weak

very strong

very weak

- 22 -

•TABLE 3. RESULTS OF HARDNESS MEASUREMENTS

Components Hardness

BEN KHN Approximate BHN

Equivalent to KHN

New grate top 269

New grate bottom 247

Used grate top 2 64

Used grate bottom 170

Material attached to

used grate bottom

Globule No. 1 340 313

Globule No. 2 167 153

New trunnion 528

Used trunnion 475

New trunnion pin 202

Used trunnion pin 12 0

New trunnion socket 340

Used trunnion socket 243

Chain link

Edge 262 240

Interior 502

- 23 -

Figure 1. Bottom view of grates as received. Smallest

scale division equals 1/16 inch.


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• • V

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8

Figure 6. Unetched microstructure of cross section

through top of the used grate. The dark

round areas are graphite nodules. X 100

Figure 7. Etched microstructure of cross section

through the bottom of the new grate. The

dark round areas are graphite nodules.

The matrix is essentially all pearlite.

Etch: 3% nital. X 100

Figure 8. Unetched microstructure of cross section

through used grate bottom showing graphite

nodules and secondary graphite. X 2 00

Figure 9. Etched microstructure of cross section

through used grate bottom showing graphite

nodules, some of which are surrounded by

secondary graphite. Matrix is nearly all

ferrite with a small amount of spheroidized

cementite. Etch: 3% nital. X 200

Figure 10. Etched microstructure of cross section

through globule no. 2 and the adjacent

portion of the grate bottom to which it

was attached. Etch: 3% nital. X 40

Figure 11. Unetched microstructure of cross section

through the new trunnion. Most of the

dark areas are graphite. X 100

%

*

Figure 12. Unetched microstructure of cross section

through the used trunnion. Most of the

dark areas are graphite. X 100

Figure 13. Etched, microstructure of cross section

through the new trunnion. Microstructure

is dendritic and consists of cementite

(white) , martensite (grey) and what appears

to be unresolved peariite (dark)

.

Etch: 3% nital. X 100
»
Figure 14. Etched microstructure of cross section

through used trunnion. Microstructure

is dendritic and consists of cementite

(white) and tempered martensite (dark)

.

Etch: 3% nital. X 100

Figure 15. Unetched microstructure of cross section

through new trunnion pin. X 100

Figure 16. Etched microstructure of cross section

through new trunnion pin. Microstructure

is "normalized" and consists of pearlite

in a pro-eutectoid ferrite matrix.

Etch: 3% nital. X 100

Figure 17. Etched microstructure of cross section

through used trunnion pin. Microstructure

consists of spheroidized carbides in a ferrite matrix. Etch: 3% nital. X 500
•1
9

«

Figure 18. Unetched microstructure of cross section

through the new trunnion socket. The

dark, round particles are graphite. X 100

Figure 19. Etched microstructure of cross section

through the new trunnion socket. The

microstructure consists of graphite

nodules (dark) , pearlite (grey) , and

massive cementite (white)

.

Etch: 3% nital. X 100

Figure 20. Etched microstructure of cross section

through used trunnion socket. Microstructure consists of graphite nodules in

a matrix of ferrite (white) , spheroidized

carbides (appearing as small dark dots) , and a small amount of pearlite.

Etch: 3% nital. X 100

Figure 21. Unetched microstructure of cross section

through used chain link. Cross section

intersects the link outside surface about

2/3 from the bottom of the photomicrograph.

There is some corrosion product or scale

(grey) at the surface. There are a few

inclusions (dark) in the material (white)

X 100
1
Figure 22. Etched microstructure of cross section

through used chain link. Cross section

intersects outside surface of link near

top of photomicrograph. Interior microstructure is primarily tempered martensite.

Arrow A indicates region of severe decarburization (mostly ferrite) near surface.

Arrow B indicates region of much less

decarburization (mostly martensite)

.

Etch: 3% nital. X 100

Figure 23. Fracture surface of one of the failed

trunnions . X 2

b

Figure 24. Fracture surface of one of the failed

chain links.

a. Dark area indicated by arrow A is

pre-existing crack. Arrow B

indicates probable origin of fracture.

X 2

b. Arrows indicate change in the

fracture plane. X 1

IRM no>-t <«A (1-7I»

5. PUBLICATION OR REPORT NO.

NBSIR 73-118

12. Gov't Accession j 3. Recipient's Accession No. Ko. 1 U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET

I. TITLE AND SUBTITLE

EXAMINATION OF FAILED COMPONENTS FROM

DISTRICT OF COLUMBIA INCINERATOR NO. 5

5. Publication Date March, 1973

Performiag Organization Code

AUTHOR(S)

T. Robert Shives

8. Pertorming Organization

NBSIR 73-118

'. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

lO. Project/ ! asky Work lime No.

31 20586

11. Contract/Grant No.

2. Spoasoring Organization Name and Address

CONSTRUCTION AND REPAIR DIVISION

Department of Environmental Services

Government of the District of Colximbia 20019

13. Typ>e of Report & Period

Covered Failure

Analysis Report

14. %>onsoring Agency Code

5. SUPPLEMENTARY NOTES

4- ABSTRACT (A 200-word or less factual summary of most significant information. If documeiu includes a significant

bibliography or literature survey, mention it here.)

A number of new and used components including grates , trunnions

,

trunnion pins, trunnion sockets, "spacers", and chain links from

District of Columbia Incinerator No. 5 were submitted for examination

by the Department of Environmental Services, District of Columbia.

Ifeny of the incinerator components were failing within a relatively

short time after installation. Examination revealed that all of the

used components except the chain links had undergone significant

mechanical property and microstructural changes when compared to the

material from the new components. These changes could be attributed

to exposure of the materials to temperatures which were excessive

for the materials employed. Material from the new and used grates did

not meet contract specifications. Two approaches to a possible

solution to the problem are discussed.

J' KEY WORDS (Alphabetical order, separated by semicolons)

Ductile iron; incinerator; mechanical properties, microstructures

;

steel; temperature.

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47

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