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Metallurgical Evaluation of

Type 304 Stainless Steel Exposed

to Woodburning Stove Flue Gases

wOhn H Smitn

U.3. DEPARTMENT OF COMMERCE

National Bureau of Standards

Center for Materials Science

Metallurgy Division

Gaithersburg, MD 2083S

f

Januarv 1 985

Issued February 1985

Final Report

P’’epared for

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.S. Consumer Product Safer/ Commission

/ashington, DC 20207
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NBSIR 85-3103

METALLURGICAL EVALUATION OF

TYPE 304 STAINLESS STEEL EXPOSED

TO WOODBURNING STOVE FLUE GASES

John H. Smith

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

Center for Materials Science

Metallurgy Division

Gaithersburg, MD 20899

January 1985

Issued February 1985

Final Report

Prepared for U.S. Consumer Product Safety Commission

Washington, DC 20207

U.S. DEPARTMENT OF COMMERCE, Malcolm Baldrige, Secretary

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director
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Introduction

Back^ound ; Metallic chimneys used for wood burning stoves have recently

been identified by the Consumer Product Safety Commission as the cause of numerous

house fires. These chimneys are presently constructed according to Underwriters

Laboratory (UL) Standard 103. Presently used chimneys are constructed of either

AISI (American Iron and Steel Institute) type 304 or type 430 stainless steel

and are either a double walled or triple walled design.

To evaluate the design of metallic chimneys and to determine the extent

of degradation of the materials of construction used in these chimneys, the Consumer

Product Safety Commission has conducted laboratory and field investigations of metallic

chimneys. The laboratory investigations consist of controlled tests in which test

coupons of sections from metallic chimneys are exposed to typical wood burning stove

flue gas environments and temperature cycles. The field investigations consist of

the examination of selected chimney sections that have been removed from service

after normal operation or after being involved in chimney fires.

Present Investigation : The objectives of this investigation were to determine

the extent of degradation in mechanical properties, to identify any microstructural

changes, and to determine the amount of corrosive attack of austenitic stainless

steels (types 304 and 321) exposed to flue gas environments and temperatures from

wood burning stove fires. In this phase of the investigation, couoons taken from

sections of new, unused metallic chimneys were exposed to flue gases generated by wood

burning stoves in controlled tests that lasted up to six months. Selected coupons

were heat treated prior to exposure to the flue gas environment to produce

a fully "sensitized” condition in the austenitic stainless steel. A limited

number of chimney sections made from type 304 stainless steel that had been

removed from field service or that were used in the laboratory test program ver •
also examined to assess the extent of any degradation during use. 1

|| I

Experimental Procedures

Materials Examined ; Test coupons were cut from new, unused sections of

double walled and triple walled chimneys made from AISI type 304 stainless steel.

The test coupons were approximately 3-inches wide by 18—inches long and were

roll formed into rings approximately 5 1 /2-inches in diameter for use in the

laboratory environmental exposure tests. The test coupons from the double

walled chimney sections were approximately 0.018-inch thick and the test

coupons from the triple walled chimney sections were approximately 0.015-inch

thick. Additional material of a stabilized grade of austenitic stainless steel

(AISI type 321) was purchased for making test coupons. These test coupons were

0.050 inch thick. This material is not currently used in the manufacture of

metallic chimneys but was included in this study to evaluate the significance

of the sensitization phenomenon in contributing to the degradation of chimneys.

The chemical composition of all material was checked and is shown in Table I.

All materials used in this test program satisfied the chemical composition

specifications for type 304 or type 321 as given by the appropriate AISI

specification.

The test coupons were evaluated in the following conditions: 1) as received

(cut from new chimney sections), 2) after a sensitization heat treatment in air in an

accurately controlled furnace for 100 hours at lOGO^F, 3) after sensitization plus !

exposure to wood burning in a stove environment for periods of 3, 4, or 6 months. The

test cycle for the wood burning stove environment exposure consisted of starting

^ a new fire each day, initially allowing it to burn hot with full air supply for

approximately 15 minutes and then controlling the temperature of the test coupon

to approximately 400“F for the remainder of the cycle. Test coupons of type 304
3

stainless steel from both double walled and triple walled chimneys were

exposed to the wood burning stove environment for 3 or 6 months after the test

coupons had been sensitized in the heat treatment furnace. Test coupons of type

321 stainless steel were exposed to the wood burning stove environment both in the

as received and in the sensitized conditions. In addition, test coupons of type

304 stainless steel from triple walled chimneys and test coupons of type 321 stainless

steel were evaluated after being sensitized in a chimney by heating with propane

for 100 hours to a temperature (measured in the test coupon) of 1000°F.

Five sections of each of two chimneys, constructed of type 304 stainless steel,

that had been in normal service for an extended time but which had not experienced

a chimney fire, were acquired for examination. In addition, sections of the

double walled chimney and of the triple walled chimney that were used in the

laboratory tests for exposing the test coupons for three and six months were

evaluated after the completion of the laboratory tests.

Description of Experimental Tests

Metallographic Examination ; Specimens were taken from all test coupons and

chimney sections to evaluate the general microstructure, to determine the presence

of sensitization, and to determine the extent of surface attack due to exposure

to the flue gases. Metallographic specimens were mounted so that the microstructure

in the through thickness direction could be examined. Additional specimens were

mounted in the more conventional manner so that the surface of the test coupons

could be examined. In addition, selected specimens were mounted as taper sections

to better identify the degree of attack approaching the surface that was exposed

to the flue gas environment. Optical metallographic analysis was performed after

polishing and etching the specimens. Most of the optical metalloeraphic analvsis

was performed at magnification of 500X, but selected examinations were done ar
4

lOOX, 250X, and 1250X as appropriate.

To examine the general microstmcture, the specimens were etched with

! glyceria (20ml HCl/5ml HN03 / 20ml glycerin) or with an etch of 15ml HCl/lOml HNO3 / j

15ml acetic acid. These etches were used to show the general microstruetural featurai

such as grain size, grain shape, and the presence of cold work. Selected specimens

were etched with Nital (5ml HN03 / 95ml alcohol) to identify the presence of any

small amounts of ferrite in the predominantly austenitic microstructure. The

glyceria etch was also used on the edge mounted specimens to show the depth and

location of attack at the surface of specimens that had been exposed to the flue

gas environments. To identify the presence and degree of sensitization, an

electrolytic etching procedure using an oxalic acid etch ( 10% oxalic acid in

water) was used. This etching procedure attacks only the carbide microconstituent

in the specimens and clearly shows the existence of sensitization by showing

the presence of carbide precipitates at the austenite grain boundaries.

Mechanical Properties Tests : Tensile test specimens were taken from

the test coupons and from the test chimneys. Tests were conducted following the

ASTM standard test method E-8-81 for Tension Testing of Metallic Materials. The

tensile test specimens had a gage section of 0.5 inches wide by 2.0 inches long

and were the thickness of the test coupons (0.015 to 0.018- inch). The 0.2%

offset yield strength, ultimate tensile strength, and percent elongation to

fracture were determined at room temperature for each test specimen. Selected

specimens were tested for hardness using the Knoop microhardness test with a

50 gram load.

Corrosion Tests : To evaluate the susceptibility to intergranular attack,

specimens were taken from the test coupons and were tested by the oxalic acid
5

etch test (the Streicher test) and by the ferric sulfate-sulfuric acid test

(the Huey test) according to procedures given by the ASTM standard practice

methods A-262-81. The oxalic acid etch test (Streicher test) is a rapid test

used for austenitic stainless steel to identify specimens that are free from

susceptibility to rapid intergranular attack caused by the presence of carbide

precipitation at grain boundaries (sensitization). Evaluation is based on

the classification of etched grain structures in the test specimen after

controlled etching with oxalic acid. The ferric sulfate-sulfuric acid test

(Huey test) is used to detect the susceptibility of austenitic stainless steels

to intergranular attack by determining the weight loss (corrosion rate) of specially

prepared specimens that are immersed in a boiling solution of ferric sulfate

and sulfuric acid. This test detects the susceptibility to intergranular attack

associated with the precipitation of chromium carbide (sensitization) in

unstabilized austenitic stainless steel. The corrosion rates measured and the

presence or absence of intergranular attack in this test is not necessarily

a measure of the performance of the material in other corrosive environments,

such as the flue gases.

Experimental Results

Evaluation of “As-Received" Material :

Chemical Composition ; Specimens from new, doubled walled and triple walled

chimneys, and from sections of AISI type 321 material in the "as received" condition

were analyzed for chemical composition. The chemical compositions are shown in

Table I. In all cases, the composition satisfied the required composition for

AISI type 304 or type 321 stainless steel.

Metallographic Analysis : All materials were examined by optical metalloera^M

r

techniques in the "as received" condition. Examination of each material was -ad^-

using aqua regia etch and a magnification of 500X to determine the general
6

microstructure, mlcroconstltuants, and grain size. An electrolytic oxalic acid

etch was used on separate specimens to determine the presence of carbides

precipitated at the grain boundaries which is indicative of sensitization. A

typical microstructure of the "as received" material taken from the double

walled chimney made from type 304 stainless steel is shown in Figure 1. This

material was fully austenitic with no evidence of residual ferrite or other

mlcroconstltuants. The grains were predominantly equiaxed with a grain size of

ASTM No. 9 (approximately 0.0006 in. diameter). Annealing twins were present in

many of the grains. The oxalic acid etch did not reveal any evidence of carbide

precipitation at the grain broundarles indicating that no sensitization had

occurred in the as received material as fabricated into a typical double walled

chimney. The structure indicated that the stainles steel was in the annealed

condition and had not been significantly cold worked.

Specimens taken from sections of triple walled chimney made from 304

stainless steel showed a generally similar microstructure to those taken from

doubled wall chimney sections. As shown in Figure 2, the microstructure consisted

entirely of equiaxed austenitic grains containing many annealing twins. No

evidence of significant cold work was observed. The grain size of the material

taken from the triple walled chimney sections was ASTM grain size No. 8

(approximately 0.0009 in. diameter) which is somewhat larger than the grain size

of the material from the double walled chimney sections. The oxalic acid etch

did not reveal the presence of any carbides at the grain boundaries which

indicates that the material in the typical triple walled chimney section had not

been sensitized.

The stabilized grade of stainless steel (type 321) was examined in the

"as received" condition. As shown in Figure 3, this material had a microstructure

similiar to the material taken from the double walled and triple walled chimney
7

sections made from type 304 stainless steel. The microstructure consisted

entirely of equiaxed austenitic grains showing a substantial amount of annealing

twins. No evidence of significant cold work was present in the microstructure

indicating that the material was fully annealed in the "as-received" condition.

The material was fine grained with a grain size of ASTM No. 9 (approximately

0.0006 in. diameter). The oxalic acid etching procedure did not reveal the

presence of any grain boundry carbides indicating the absence of sensitization

in the material.

Mechanical Properties Tests t The results of the tensile tests performed

on the "as received" material from double walled and triple walled chimney

sections are shown in Table II. No tensile tests were performed on the

stabilized stainless steel (AISI type 321). The results show that the

yield strength, tensile strength, and percent elongation to fracture of

material from both the double walled and triple walled chimney sections fall

within the normally expected range for annealed type 304 stainless steel.

The yield strength and tensile strength of the material from the triple

walled chimney section were slightly lower that the same properties for

material from double walled chimney sections. This small effect may be

due to the somewhat larger grain size in the triple walled material than

in the double walled material.

Corrosion Tests ; Specimens of all "as received" materials were tested usin^

the Streicher test (ASTM method A-262-81, practice A) which is an oxalic acid etch

test for classification of austenitic stainless steels to determine their susceptibility to intergranular corrosion. The three materials tested, which were from double

walled and triple walled chimneys and from type as received 321 stainless

steel, all showed a satisfactory etched structure classified as a "step" structur .
On the basis of the Streicher test these materials are not susceptible

to intergranular attack due to carbide precipitation (sensitization) at the

grain boundaries*

Additional corrosion tests were conducted on material from the double

walled and triple walled chimney sections using the ferric sulfate-sulfuric acid

(Huey) test (ASTM method A-262-81 Practice C) for determining the susceptibility to

intergranular attack. This procedure determines the corrosion rate of stainless

steel in boiling sulfuric acid. The measured corrosion rate for both materials

was 26 to 29 mpy (mils per year) which indicates that the stainless steels I

were not susceptible to intergranular attack in the sulfuric acid environment. '

Evaluation of Sensitized Material :

Test coupons of type 304 stainless steel from both double walled and triple

walled chimney sections and of type 321 (stabilized) stainless steel were heat *

treated in a furnace for 100 hours at 1000®F to cause carbide precipitation at the grain^

I boundaries (sensitization) prior to being exposed to wood burning stove environments.

In addition, selected coupons from a triple walled chimney and from sections

of type 321 stainless steel were heated in a wood burning stove chimney

for 100 hours at a controlled temperature of 1000® F to cause sensitization.

Metallographic Analysis: As shown in Figure 4, the sample of material from

the double walled chimney section showed severe sensitization after the heat

treatment as indicated by the carbide precipitation completely surrounding the

grains. The degree of sensitization was most extreme at the surface of I

the specimen and decreased somewhat in the interior (mid thickness) of the

specimen. As shown in Figure 5 , at a magnification of 500X the surface of the

test coupon is slightly rough after the completion of the heat treatment, but

| no evidence of grain boundary attack is apparent. The features of this
9

mlcroRraph (Figure 5) will be compared with similar micrographs taken after

exposure of the specimens to the wood burning stove environment. No changes

in the microstructure, other than the carbide precipitation at the grain

boundaries, were found after the sensitization heat treatment (for example,

compare Figure 1 and 4).

As shown in Figure 6 , samples taken from triple walled chimney sections

showed evidence of extreme sensitization after the heat treatment at 1000® F

for 100 hours in the furnace. The degree of sensitization in this material

was more extreme at the surface of the specimen than of the mid-thickness of

the specimen as shown in Figure 7. Complete sensitization was also observed

in the test coupons that were heated in the wood burning stove chimney for

100 hours at 1000° F, as shown in Figure 8. The surface of the test coupons

were only slightly roughened and no grain boundary attack«was observed after the

furnace heat treatment, as shown in Figure 9. The carbide precipitation

due to sensitization was the only microstructural change observed in the

material from the triple walled chimney sections after the heat treatment.

Samples of the type 321, stabilized grade of stainless steel were examined

after heat treating for 100 hours at 1000° F in both the heat treating furnace

and in the wood burning stove chimney. As shown in Figure 10, the type 321

stainless steel was not sensitized by this heat treatment as indicated by the

lack of carbide precipitation at the grain boundaries. This is in contrast

to the samples of type 304 stainless steel (unstablized) that were heavily

sensitized after the same heat treatment.
10

Mechanical Properties Tests: Tensile tests were conducted on samples of

materials from double walled and triple walled chimney sections after the

materials were heat treated In the furnace for 100 hours at 1000® F. The

results of these tests, given In Table II, show that no significant change

occurred In the yield strength, tensile strength, or percent elongation to

fracture as a result of this sensitization heat treatment. The mechanical

properties of the sensitized 304 stainless steel were with the normally

expected range of property values for annealed type 304 stainless steel.

Corrosion Tests ; The samples of materials from both the double walled

and triple walled chimney sections tested In the Strelcher test both showed

severe evidence of sensitization (etching at the grain boundaries after the

oxalic acid etch) after the materials had been heat treated for 100 hours

at 1000° F. Specimens of sensitized materials tested by the Huey test

method showed corrosion rates varying from 162 mpy for the materials from

triple walled chimney sections to 420 mpy for materials from the double walled

chimney. These much higher corrosion rates, compared with rates of 26 to

29 mpy in the "as received" material, indicate that severe and rapid intergranular

corrosion took place in the sensitized material when tested in the sulfuric acid

environment.

Evaluation of Material After Exposure to Wood Burning Stove Envlronmemts:

Test coupons were tested in wood burning stove chimneys for periods of 3 months

and 6 months according to the temperature and firing cycle previously described.

The test coupons were examined metallographically and tensile tests and

corrosion tests were conducted on the materials after the 3 months exposure

period. All material was sensitized prior to exposure to the wood burning

environment
11

Metallogr-aphlc Exam-t nation ; Specimens of type 304 stainless steel material

from double walled chimney sections showed no change in microstructure or in

the degree of sensitization after exposure to the wood burning stove environment

for 3 months. Several specimens from each of six coupons placed in different

positions in the chimney were examined. The only evidence of any change in the

material is the very slight surface roughening and grain boundary attack that

can be seen at high magnification (1250x) as shown for example in Figure 11. The

maximum extent of this attack is to a depth of one grain (approximately 0.0006 in).

After six months exposure, the same materials show no additional micros tructural

change from the condition of the starting (as sensitized) material. As shown

in Figure 12, the attack at the surface of the specimen is still limited to a

maximum of one grain diameter in depth. No severe attack or degradation was

observed at the grain boundary which” indicates that the presence of carbides

in the grain boundaries due to sensitization does not significantly affect the

degradation of the type 304 stainless steel in the flue gas environment.

Specimens of materials taken from triple walled chimney sections showed

no significant change in microstructure or in the degree of sensitization after

3 months or 6 months exposure to the wood burning stove environments. After

3 months exposure, the maximum extent of attack was limited to the depth of one

grain diameter and could only be observed at high magnification as shown in

Figure 13. After 6 months exposure, the extent of attack in the surface of the

specimens was not significantly greater than after 3 months exposure. As shown

in Figures 14 and 15, the surface attack was limited to a depth of approximately

one grain diameter (0.0009 in ). This indicates that the rate of surface degration and attack is not accelerating as the exposure times are extended.
12

The type 321 (stabilized) stainless steel test coupons were exposed to the

'

wood burning stove environment for a period up to 4 months after first being

j heat treated through the cycle that was used to sensitize the 304 stainless steel.

| As shown in Figure 16, no microstructural changes were observed in the type i

321 stainless steel as a result of the exposure cycle. The only evidence of

surface degradation or attack was limited to a depth of less than one grain

diameter (0.0006 in.), as shown in Figure 16.

The results of the metallographic examination show that the wood burning stove

heating cycle used in these tests does not cause any microstructural changes and doesj

not cause significant degradation or attack of the surface of type 304 stainless

steel even when the stainless steel is heavily sensitized prior to exposure to : !

the heating cycle. Type 321 stainless steel, which is not subject to sensitization,

does not show any difference from the type 304 stainless steel when exposed to the

wood burning stove envrionment.

Mechanical Properties Tests : Tensile tests were conducted on samples of

materials from double walled and triple walled chimney sections that had been

exposed to wood burning stove environments for a total time of 3 months. The

results of these tests, given in Table II, show that no significant change

occurred in the yield strength, tensile strength, or percent elongation to f!

fracture as a result of the exposure to the wood burning stove environments.

The mechanical properties of the 304 stainless steel were within the normally

expected range of property valves for annealed type 304 stainless steel.

Corrosion Tests: Specimens from both double walled and triple walled

i chimney sections were tested using the Streicher test after 3 months exposure ffto wood burning stove environments. This test, which evaluates the susceptibility

of austenitic stainless steel to intergranular cracking, showed that both the
13

double walled and triple walled chimney sections were susceptible to Intergranular

corrosion. However, these tests showed that the susceptibility to this type

of corrosion was not Increased by the exposure to the wood burning stove

envlronmemt because the corrosion specimens gave the same results after exposure

as they had In the sensitized condition prior to the 3 month exposure.

Specimens from both double walled and triple walled chimney sections were

tested further using the Huey test to determine the corrosion rate In a ferric

sulfate-sulphuric acid solution. The corrosion rate ranged from 369 to 420 mpy

for the double walled specimens and from 55 to 162 mpy for the triple walled

specimens. These results were the same as for these materials after sensitization

but prior to exposure for 3 months In the wood burning stove environments.

Therefore, It Is concluded that the sensitization heat treatment Increases the

susceptibility to Intergranular corrosion but that exposure to the wood burning

stove environment after sensitization does not further Increase the susceptibility

of these materials to Intergranular corrosion.

Evaluation of Type 304 Chimneys Removed From Service : Two entire chimneys

were removed from service after an unspecified period of use. These two

chimneys were not reported to have experienced any chimney fires. These

chimneys, identified by CPSC sample numbers as F-815-3688 and F-815-3689, were

both double walled chimneys manufactured by Selkirk Metalbestos Inc. and

designated as model 7-SS-30. Five sections of each chimney were obtained.

These sections were identified by their location in the chimney, with section

one being at the stove end of the chimney and section five being the section above

the roof of the house.
14

Only the inner liner of each chimney section was examined metallographically. T

Samples from sections nearest the stove and nearest the roof from each chimney .

i

I

! were examined to determine the presence of sensitization. Figure 17 shows a

“|

representative microstructure after etching with oxalic acid to detect sensitization. ,

No evidence of sensitization was found in the sections of these chimneys which were i

examined. This indicates that the temperature did not remain for sufficient time

in the appropriate temperature range for sensitization to occur (800° F to 1200° F)

during normal operation of these chimneys.

Additional samples were taken from each of the five sections from each

chimney and examined by optical metallography to determine the extent of any

microstructural changes and any corrosive attack during normal operation

of the chimneys. Figure 18 shows the typical microstructure from the chimney

sections. These chimney sections show a micros tructure typical of annealed

type 304 consisting of equiaxed grains of austenite containing annealing

twins. No significant corrosive attack was found on the surface of the

specimens. This examination did not show any significant microstructural

changes as a result of the operation of the chimneys.

The two chimneys, one double walled design and one triple walled design,

which were used in the CPSC laboratory for exposure of test coupons for 3 and

6 month tests were examined metallographically. The two sections of each

chimney nearest the wood burning stove were obtained for evaluation. Specimens I

fi from the inner liner of the chimney sections were examined for evidence of j[>

sensitization due to the temperature cycle used in the exposure tests. As 3

shown in Figure 19, no evidence of sensitization was found. This indicates V

that normal wood stove operating cycles will not cause sensitization of the
15

type 304 stainless steel used to fabricate the chimney. Specimens of both

double walled and triple walled chimney sections were also examined to assess

the extent of any other microstructural changes or corrosive attack that occurred

to the inner liner of the chimneys during the six month exposure tests. Figure

20 is representative of both the doubled walled and triple walled chimney section

specimens and shows that the microstructure consists of equiaxed grains of austenite,

typical of annealed type 304 stainless steel. No significant corrosive attack

is noted at the edge of these specimens as a result of the exposure for 6

months to the wood burning stove environment.

Discussion ;

Test coupons of type 304 and stabilized type 321 stainless steel were

exposed to wood burning stove environments in a typical temperature cycle

experienced for normal stove operation for up to six months. All of the type

304 material was heavily sensitized prior to exposure in contrast to the type

321 alloy which is not subject to sensitization. The as-received material

for all test coupons and for all chimneys evaluated was typical of annealed,

austenitic stainless steel. The mechanical properties, strength and ductility,

of the type 304 were not significantly affected by the sensitization or by the

exposure to the wood burning stove environments. The material from the

particular triple walled chimney sections evaluated had a slightly higher

ductility and lower strength than the material from double walled chimney

sections. This effect may be attributed to the smaller grain size in the

material from double chimney sections than in the material from triple

walled chimney sections. No change was observed in the microstructure of any

of the materials as a result of the exposure to the wood burning stove
16

environments and to the temperature cycles used in this test program. A

minimal amount of surface attack, limited to less than one grain diameter

in depth, was the only observed change during the exposure tests or in

chimney sections removed from normal service. Sensitized material did not

perform significantly differently in these tests than the nonsensitized

(stablized) type 321 stainless steel.

Under conditions which may occur in a wood buiming stove chimney, the

most likely changes in microstructure or mechanical properties in type

304 stainless steel are due to sensitization and oxidation. Sensitization

will occur if the stainless steel is heated or slowly cooled within the

temperature range of approximately 800° F to 1500° F. This will cause

the precipitation of carbides at the grain boundaries which is known to

Increase the rate of corrosion in an aqueous environment. However, as

shown in the tests reported here, sensitization does not result in increased

attack in the dry, flue gas environments from the wood burning stove. The

effect of sensitization on the high temperature oxidation rate was not investigated

here.

The type 304 stainless steel may be subject to degradation by high temperature

oxidation in the event of a severe fire in the chimney or due to sustained operation

at temperatures in excess of 1500° F, No evidence of significant oxidation nor K

other attack was found in the present tests in which the temperature cycle was 1;

ii controlled within the normally expected range for a well operated wood burning stove. I;

i

Conclusions

From an evaluation of the mechanical properties and microstructure of

austenitic stainless steel chimney liners, the following conclusions are reached:
17

1. The austenitic, type 304, stainless steel test coupons did not

show significant degradation and did not show any change in

microstructure as a result of exposure for up to 6 months to wood

burning stove environments and temperatures used in these tests.

2. No significant change in mechanical properties, such as embrittlement, was

found in the type 304 stainless steel as a result of sensitization or

exposure to particular controlled wood burning stove environments used in

these tests.

3. Sensitization prior to exposure did not increase the amount of degradation

of type 304 stainless steel in the particular controlled wood burning

stove environments used in these tests.

4. Stabilized stainless steel, type 321, did not perform significantly

differently than sensitized type 304 in the exposure tests.

Suggested Future Work ;

1. Evaluate the presently used test procedures to determine if they

adequately assess the design integrity of the chimneys under normal

operating conditions and reasonably expected temperature cycles.

2. Evaluate the design and materials degradation of chimney parts

other than the inner liners, for example, the spacer brackets.
IIIiir!\II11i

I'
TABLE I

Chemical Composition (percent)

Material £ Mn _P _S Si Cr Ni Other

304 Double Walled 0.06 1.80 0.033 <.005 0.61 19.03 8.43

304 Triple Walled 0.06 1.53 0.024 0.007 0.54 18.75 8.17

304 Specification* 0.08 2.00 0.045 0.030 1.00 18.00 8.00

max. max. max. max. max. 20.00 10.50

321 Test Coupons 0.05 1.64 0.022 <.005 0.74 17.00 10.61 Ti=0.38

321 Specification* 0.08 2.00 0.045 0.030 1.00 17.00 9.00

max. max. max. max. max. 19.00 12.00 Ti=5xC min.

* From American Iron and Steel Institute Steel Products Manual
TABLE II

304 Stainless Steel

( 1 ) Tensile Test Results

0.2% Offset Tensile

Material Yield Strength (KSI) Strength (KSI) % Elongation

Type 304

Type^ 304

Double Walled:

As Rec'd. 46.8 102.0 44.1

Sens.* 44.8 101.9 37.7

Exposed** 45.8 102.2 47.2

Triple Walled:

As Rec*d. 39.9 92.8 52.4

Sens.* 38.4 93.2 50.1

Exposed** 40.8 96.1 56.4

Typical Annealed Type 304:

35-50 80-105 40-60

*Sensitized at 1000°F for 100 hours

**3-month exposure in CPSC lab tests

(1) Each value represents average of four tensile tests
As-Received Double Walled Material 500X

Fig. 1

As-Received Triple Walled Material 500X

Fig. 2

As-Received Type 321 Stainless Steel 500X

Fig. 3

Double Walled Material After Sensitization 600X

Fig. 4

Edge of Double Walled Material 500X

After Sensitization

Fig. 5

Triple Walled Material After Sensization SOOX

Fig. 6

Through Thickness Section of Triple Walled 200X

Material After Sensitization

Fig. 7

Triple Walled Material - Chimney Sensitized 500X

Fig. 8

I.

Edge of Triple Walled Material 500X

After Sensitization

Fig. 9

Type 321 Stainless Steel After Sensitization Heat Treatment

Fig. 10

Double Walled Material After Exposure 1250X

Fig. 11

Edge of Double Walled Material After Exposure 500X

Fig. 12

Triple Walled Material After Exposure 1250X

(As Polished)

Fig. 13

Edge of Triple Walled Material After Exposure 500X

Edge of Triple Walled Material After Exposure 500X

Fig. 15

Type 321 Stainless Steel After Exposure 500X

Type 304 Chimney Section from Field Showing Carbides 500X

Fig. 17

Edge of Type 304 Chimney Section from Field SOOX

Fig. 18

Type 304 Chimney from Lab Tests 500X

Showing Carbides

Fig. 19

Edge of Type 304 Chimney Section 500X from Lab Tests

Fig. 20

N8S-n4A 1 REV. 2 -ac) U.S. DEPT. OF COMM. 1. PUBLICATION OR 2. Performing Organ. Report No. 3. Publication Date BIBLIOGRAPHIC DATA

SHEET (See instructions)

REPORT NO.

NBSIR 85-3103 April 1905

4. TITLE AND SUBTITLE

I^TALLURGICAL EVALUATION OF' TYPE 30U STAINLESS STEEL EXPOSED TO WOODBLNUJING STOVE

FLUE GASES

5. AUTHOR(S)

John H. Smith

6. PERFORMING ORGANIZATION (If joint or other than N3S, see instructions)

national bureau of standards

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

7. Contract/Grant No,

8 . Type of Report & Period Covered

Final

9. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street. City. State, ZIP)

Engineering Laboratory

Directorate for Engineering Sciences

Consumer Product Safety Commission

5^01 Westbard Avenue, Washington, DC 20207

10. SUPPLEMENTARY NOTES

I I Document describes a computer program; SF-185, FIPS Software Summary, is attached.

11. ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a si gn i fi cant bibliography or literature survey, mention it here)

A metallurgical evaluation vas conducted to evaluate the susceptibility of Tj’pie

304 Stainless Steel to degradation in woodburning stove flue gas envirorunent s . The

Type 30U Stainless Steel did not show significant degradation when exposed to flue

gas environments for up to six months. No significant change in the degree of

degradation was found when the 30^+ stainless steel was sensitized.

12. KEY WORDS (Six to twelve entries; alphabetical order; capitalize only proper names; and seporote key words by semicoions

Austenitic stainless steel; corrosion; flue gas; metallic chimney; stainless sT e.

woodburning stove chimney

13. AVAILABILITY

i i Unlimited

[~X] For Official Distribution. Do Not Release to NTIS

I I Order From Superintendent of Documents, U.S. Government Printing Office, Washington. D.C. 20402.

I

i Order From National Technical Information Service (NTIS), Springfield, VA. 22161

14. NO. OF

PRINTE:

15. Pri c«


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i.