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A11101 5 b 5 M b ^ NBSIR 81-2394 &)

Metallurgical Evaluation of

Seamless Stainless Steel Cylinder

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U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

National Measurement Laboratory

Center for Materials Science

Fracture and Deformation Division

Washington, DC 20234

September 1981

Final Report

Issued November 1981

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N8SIR 81-2394 c

**' Q.C. t0~0 METALLURGICAL EVALUATION OF

SEAMLESS STAINLESS STEEL CYLINDER

John H. Smith

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

National Measurement Laboratory

Center for Materials Science

Fracture and Deformation Division

Washington, DC 20234

September 1981

Final Report

Issued November 1981

This work was sponsored by

Department of Transportation

Washington, DC 20590

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

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director
.
Summary

A seamless cylinder fabricated from type 30^ stainless steel was

examined to assess the uniformity of properties and microstructure and

to determine if the cylinder was in compliance with the DOT regulation

governing these cylinders. The properties and microstructure did not

show excessive variation throughout the cylinder. However, some evidence

of carbide precipitation and sensitization were found in the cylinder.

Recommendations are made to prevent possible sensitization in cylinders

of this type.

Metallurgical Evaluation of Seamless

Stainless Steel Cylinder

1. INTRODUCTION

1.1 Reference II]

Office of Hazardous Materials Regulations, Department of Transportation,

Washington, D.C. letter from A. J. Mallen to J. H. Smith, NBS. April IT,

1980, reference contract D0T-A-S-40034.

1.2 Background

Sections of a seamless cylinder from a lot of 12 cylinders manufactured

by Autospin Inc., Carson, California were supplied "by the Department of

Transportation (DOT)" to NBS for metallurgical and mechanical properties

evaluation. The cylinders in the lot were 2. 000- inches (5.080 cm)

outside diameter, l6. 250- inches (41.275 cm) long and had a minimum wall

thickness of 0.l83-inch (.0.465 cm).. The cylinders were made from seamless

tubing of type 304 stainless steel (.ASTM designation A511) and were made

to comply with the DOT specification No. DOT-3A-500 C49 CFR part 178. 36).

The cylinders were manufactured by hot spinning the ends on sections of

annealed, seamless, stainless steel tubing. The hot spinning was reported

to have been done at a temperature of l600°F (871°C) and was followed by

water quenching.

The cylinders had a capacity of approximately 500 C.C. (30.5 cu. in.) and were designed to operate at a pressure of approximately 500 psi

(3448 KPa). and were hydrostatically tested after manufacture to a maximum

pressure of 833 psi (3744 KPa). At the test pressure, the calculated

wall stress was 3926 psi (27070 KPa I, based on 49 CFR part 178. 36-10.

The maximum allowable wall stress 0+9 CFR part 178. 36-10) for a type

D0T-3A cylinder is 2U,000 psi (165493 KPa)..

As specified in the DOT regulations (.49 CFR part 178. 36-11 ), all

seamless cylinders must be uniformly and properly heat treated after

fabrication and prior to conducting the required physical tests. The

inspection report Cl) shows that the lot of 12 completed cylinders, from

which the test cylinder evaluated here was taken, was not heat treated

after fabrication and prior to conducting the physical tests. However,

the inspection report states that, because austenitic stainless steel,

such as the type 304 used here, will not increase in strength during

heat treatment, the final heat treatment requironent may not be necessary.

The cylinders- in this lot did not strictly comply with the DOT

regulations which require uniform heat treatment after fabrication.

Therefore, it was decided to assess the significance of this lack of

final, heat treatment by determining, l) the uniformity of the metal

properties and 2). if the metallurgical structure is essentially the same

as that which would result from heat treating the completed cylinders

( 11 .

- 1 -
2. EXPERIMENTAL WORK AMD RESULTS

2.1 Evaluation of the Uniformity of Properties

To evaluate the uniformity of the completed cylinder, the cylinder

vas sectioned longitudinally and hardness measuraaents were made on the

necx, sidewall, and hottcm oi the cylinder. The hardness results are

shown in Figure 2.. These results show that the hardness is quite uniform

throughout the cylinder and is typical of that of annealed stainless

steel of type 30U. The hardness of annealed 30U stainless is specified

to he in the range of 75 to 90 on the Rockwell B scale (2) variation

from a hardness of 88 Rg at the cylinder neck to a hardness of 8U Rg

along the cylinder sidewall were observed. This range of hardness is

within the normally expected variation for a uniform wrought stainless

steel product and indicates that the cylinder was essentially annealed

with, only a very minimum amount of cold work during fabrication of the

cylinder. The bottom of the cylinder, which was hot snun and water

quenched, exhibited hardness- levels of 78 Rb to 85 Rb* These hardness

levels are slightly below the hardness levels found in the sidewall and

neck of the cylinder. This indicates- that some slight softening due to

annealing and possibly grain coarsening occurred during the hot spinning

of the cylinder bottom. For comparison, a sample was cut from the

sidewall of the cylinder, fully annealed for 1 hour at 2000OF ( 10930c)

and water quenched. This- fully annealed sample had hardness measured in

the range 73 Rb to 77 Rb* This- indicates that the bottom of the cylinder,

which had a hardness- of J8 Rg to 82 Rg, was essentially fully annealed.

Overall, the hardness- measurements, which ranged from 78 Rb to

88 Rg throughout the completed cylinder, indicate that the properties of

the metal in the cylinder are only slightly more variable than would be

expected if the cylinders- were fully heat treated by annealing after

completion of the fabrication of the cylinder. As indicated by the

fully annealed test sample, further annealing might lower the hardness

even further to the 73 Rb "to 77 Rb range due to grain coarsening.

2.2 Evaluation of the microstructure

Optical photomicrographs were taken at several locations throughout

the cylinder to assess- the uniformity of the microstructure. The locations

on the cylinder at which photomicrographs- were taken are shown in Figure 3*

The mi.crostructure shows a slight amount of cold work at the neck

of the cylinder as indicated by slightly distorted and elongated grains

(Figure b and 5, Photomicrographs A and 3). The remainder of the cylinder

has a microstructure that is primarily equiaxed grains typical of annealed

stainless steel (Fig. 6 , J, 8 , 10 , 11, 12, 13 corresponding to locations

C, D, E, F, G, E, T, -J of Fig. 3 1* The microstructure at the bottom of

the cylinder (Fig. 12 and 13 1, which was formed by hot spinning, shows

somewhat larger equiaxed grains than the rest of the cylinder which

indicates that some grain coarsening occurred during the hot spinning.

2
2.3 Evaluation of susceptibility to carbide precipitation and investigation

Type 30k stainless steel is known to be susceptible to a phenomena

known as sensitization which significantly lowers its resistance to

intergranular corrosion (,3l. Sensitization occurs due to the precipitation

of carbides at the grain boundaries when the steel is held at temperatures

between 800°F (U27°C) and l600°F (871°C) or when it is cooled slowly

through this temperature range. The presence of sensitization will not

be detected by changes in properties, such as hardness, but can be

detected from examination of the microstructure by etching with oxalic

acid (_U) to highlight the presence of carbides in the grain boundaries.

Photomicrographs taken at the bottom of the cylinder (Fig. 12,

location I). shewed no carbide precipitation and sensitization. This

indicates that the bottom of the cylinder which was hot spun, was heated

sufficiently (above approximately l600°F/871°C ) during hot spinning and

was properly water quenched so that the carbon remained in solution in

the grains and did not precipitate and caus-e sensitization.

Photomicrographs taken along the sidewall of the cylinder but near

the bottom (Jig. 8, 9» 10 at locations E, F, G) did show evidence of

carbide precipitation at the grain boundaries. This indicates that

although the bottom of the cylinder was heated sufficiently to avoid

sensitization, the sides of the cylinder a short distance back from the

bottom were heated by conduction into the critical temperature range to

cause sensitization.

To assess the susceptibility to sensitization, sections of the

cylinder were reheat treated to simulate the range of heat treatments

that the cylinders might receive. The following heat treatments were

performed:

Temperature Cooling Objectives

A. 20QQ°F/1093°C Water Quench 1 .

2.

To demonstrate full and proper

annealing

To show absence of sensitization

B. 1200°F/6U9°C Air Cooled 1 . To demonstrate worst case of

sensitization

The photomicrographs from

treatment A and in Figure

these tests are shown

15 for heat treatment

in Figure lh for heat

3. As expected, these

tests shew that if the cylinder is fully and properly annealed by

heating to 200°F (1093°C). and water quenched, all evidence of sensitization

is absent. However , if heated within the critical temperature range of

30Q°F (>27°C) to l600°F (.871°C )_ and slow cooled, severe sensitization

will occur.

- 3 -
3. DISCUSSION

3.1 Uniformity of Properties

The hardness measurements taken throughout the cylinder that was

supplied, indicated that the properties of the cylinder were generally

uniform and were within the range expected for annealed 30U stainless

steel (2). The hardnes-s at the bottom of the cylinder, which had been

hot spun and water quenched, was slightly lower than the hardness of the

rest of the cylinder due to grain coarsening. The specimen that was

taken from the cylinder and fully annealed had a hardness in the range

of 73 Rg to 77 Rg» which indicates- that the entire cylinder may have

been slightly cold worked during fabrication (hardness ranged from 78 RB

to 88 Rg)_. However, any cold working was not sufficient to be detected

metallographically. Therefore, a final heat treatment of the cylinder

for purposes of annealing would not result in any significant change in

the properties of the cylinder or any substantial improvement in the

uniformity of the properties.

3.2 Uniformity of the Metallurgical Structure

The microstructure was essentially equiazced grains throughout the

cylinder representative a fully annealed micro structure. A very limited

amount of cold working was- evident at the neck of the cylinder. Slightly

enlarged, equiaxed grains were obs-erved in the hot spun bottom of the

cylinder. However, this amount of grain coarsening is not expected to

result in significant degradation of the strength or other mechanical

properties of the cylinder. For comparison, the sample of the cylinder

that was reheat treated by annealing at 2000OF (1093°C ) and quenched in

water to simulate fully annealing the cylinder shows grain coarsening

similar to that observed in the bottom of the cylinder (Fig. ±k\. This

annealing resulting only slightly lowering the hardness to a range of

73 RB to 77 R3

.

3.3 Evaluation of the need for Uniform Heat Treatment

As discuss-ed above., the properties and microstructure of the cylinder

are nearly uniform throughout the completed cylinder. Additional heat

treatment by annealing would not substantially change the properties or microstructure. At most any annealing would only slightly soften the

cylinder and cause some coarsening of the grain structure.

However, annealing followed by water quenching would remove any

carbide precipitation at the grain boundaries which leads to sensitization

and can cause a significant decrease in the corrosion resistance of the

cylinder. Some sensitization was- observed in the sidewall of this

cylinder just above the hot spun bottom. The carbide precipitation and

sensitization is a reversible process and is removed by annealing of the

stainless sbeel at temperature above 1600OF (871°C )_ followed by rapid

cooling (water quenching!. As shown by Figure lU, all traces of sensitization

were removed by a proper anneal of a section of the cylinder. Therefore,

if the fabrication of the cylidner causes it to be heated within the

critical temperature range of 800°F ( l27°C 1 to l600°F (871°C)' where

sensitization can occur, it is- recommended that a final annealing of the

complete cylinder be performed.

- 4 -
3.^ Assessment of the Adequacy of the Cylinder Design and Material

The present cylinders are designed to operate at a pressure of

500 psi (3^8 KPa) and to be proof tested at a pressure of 833 psi

(57^ KPa). At the proof test pressure of 833 psi ( 57 Uh- KPa) the calculated

wall stress is 3926 psi (27,070 KPa). The measured minimum hardness on the cylinder was 78 Rg which corresponds to a yield strength of approximately

30,000 psi (206,867 KPa) and a tensile strength of 75,000 psi (517167 KPa).

This indicates that even at the proof test pressure, the wall stress is

less than 1/7 of the minimum yield strength of the cylinder, so that

there is a very large margin of safety against failure by burst. In

addition, the elongation to fracture was reported to be bb% (l), which

indicates very high ductility and resistance to fracture.

b. CONCLUSIONS

b.l The stainless steel cylinders as fabricated are adequate for their

intended use. The microstructure and properties do not show excessive

variation throughout the cylinder. A wide margin of safety against

failure by bursting exists because the wall stress, even at the proof

test pressure, is less than 1/7 of the yield strength of the material in

the cylinder.

b .2 The cylinder showed some evidence of sensitization in a limited

region which, can result in an increase in the susceptibility to intergranular

corrosion. This sensitization cannot be detected by hardness measurements

or during proof testing. The sensitization, when it occurs can be

removed by fully annealing the cylinder after fabrication.

5. RECOMMENDATIONS

5.1 The DOT regulations as presently written (.19 CFR part 178.36)

specify that uniform and proper heat treatment should be done after the

cylinder is fabricated and prior to conducting the physical tests. It

would be more correct if the regulations were revised to distinguish

between cylinders requiring heat treatment for the purpose of increasing

the strength of the metal, such as carbon steels, and cylinders that are heat treated by annealing, such as austenitic stainless steel cylinders,

for the purpose of increasing the ductility and corrosion resistance of

the cylinder. Por cylinders that are strengthened by heat treatment, a uniform and proper heat treatment should be mandatory. Cylinders made

from metals that are not strengthened by heat treatment, should only be

required to meet satisfactory strength and ductility requirements.

5.2 For cylinders- made from austenitic stainless steel, the DOT regulations

should be revised to minimize or eliminate the possibility of sensitization

during fabrication. This- may be done by restricting the fabrication of

these cylinders to stabilized grades of austenitic stainless steel such

as types 321 or 3^-7, or to extra low- carbon grades such as types 30^-L or 316L. If type 30*+ or 316' stainless steel is permitted for these cylinders

,

a proper heat treatment to a temperature of at least l600°F (871 C)

followed by water quenching should be required to remove any trace of

sensitization after fabrication.

- 5 -
REFERENCES

Memorandum, April IT, 1980 from A. J. Mallen, Office of Hazardous

Materials Regulations, Dept, of Transportation to J. H. Smith,

National Bureau of Standards.

"Aerospace Structural Metals Handbook, " Mechanical Properties Data

Center, Belfour Stelun Inc., Traverse City Michigan, 1975 ed..

Section 1303 pg. 1-29.

"Source Book on Stainless Steels," American Society for Metals,

Metals Park, Ohio, 19.76, pg. 162.

"Standard Recommended Practices for Detecting Susceptibility to

Intergranular Attack in Stainless Steel" ASTM Standard A-262.
Fig.

1.

Sections

of

Cylinder

As

Received.

88

85

85

85

85

84

84

84

84

FIG. 2 HARDNESS MEASUREMENTS (ROCKWELL B SCALE)

FIG. 3 LOCATION OF PHOTOMICROGRAPHS

Fig. h. Photomicrograph at Location A shoving

slightly deformed grains. Mag. 100X

Fig. 5 Photomicrograph at Location 3

showing slightly deformed

grains. Mag. 100X

Fig. 6 Photomicrograph, at Location C

shewing ecruiaxed grain structur

Mag. 100X

Fig. 7* Photomicrograph at Location D

shoving typical ferrite stringer.

Mag. 500X

Fig. 8. Photomicrograph at Location 2

shoving carbide precipitation at

grain boundaries. Mag. 100X

Fig. 9 Photomicrograph, at Location P

shoving equiaxed grain structure

Mag. 100X

Fig. 10. Photomicrograph, at Location 0

showing carbide orecipitation.

Mag. 100X

Fig. 11. Photomicrograph at Location H

showing equiaxed grains. Mag. 200X

[B 1 'W*'

k -v Ov \ //If' i

’ai' til .

* |ik t 7 r ~T

Fig. 12. Photomicrograph. Location I shoving no carbide

precipitation. Mag. iQOX

Fig. 13 Photomicrograph Location J

shoving enlarged equiaxed

grains. Mag. 100X

Fig. lU. Photomicrograph. of reannealed

specimen 2000°F /water quenched.

Mag. 100X

NBS«I14A (rev. 2»ac) U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET (See instructions)

1. PUBLICATION OR

REPORT NO.

NBSIR 81-2394

2. Performing Organ. Report No. 3. Publication Date November 1981

4. TITLE AND SUBTITLE

METALLURGICAL EVALUATION OF SEAMLESS STAINLESS STEEL CYLINDER

5. AUTHOR(S)

John H. Smith

6. PERFORMING ORGANIZATION (If joint or other than N BS. see instructions)

national bureau of stanoaros

DEPARTMENT OF COMMERCE

WASHINGTON. O.C. 20234

7. Contract/Grant No. DOT- A/S 4003^

S. Type of Report & Period Covered

Final

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

Office of Hazardous Materials, Regulation Materials Transportat ion Bureau

U.S. Department of Transportation

400 7th Street, S.W.

Woghjngotn. DC 20590

10. SUPPLEMENTARY kOTES

| 1 Document describes a computer program; SF-185, FIPS Software Summary, is attached.

11. ABSTRACT (A 200-word or less factual summary of most significant in formation. If document includes a significant

bi bliography or literature survey, mention it here)

A seamless cylinder fabricated from type 304 stainless steel was examined to

assess the uniformity of properties and microstructure and to determine if the

cylinder was in compliance with the DOT regulation governing these cylinders.

The properties and microstructure did not show excessive variation throughout

the cylinder. However, some evidence of carbide precipitation and sensitization

were found in the cylinder. Recommendations are made to prevent possible

sensitization in cylinders of this type.

12. KEY WORDS (Six to twelve entries; alphabetical order; capitalize only proper names; and separate key words by semicolon s) .Metallurgical evaluation; Pressure vessels; Stainless steel; Stainless

steel, austenite.

13. AVAILABILITY

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20402.

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15. Price

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