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Monitoring Valve Health via the Internet

Monitoring Valve Health via the Internet

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Valves in Oxygen Service

Valves in Oxygen Service

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Thermal Spray Coating

Thermal Spray Coating

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Ball Valve Repair 101

Ball Valve Repair 101

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Industry Headlines

LyondellBasell to Build the World's Largest PO/TBA Plant

Friday, 21 July 2017  |  Chris Guy

LyondellBasell has made the final investment decision to build the world's largest propylene oxide (PO) and tertiary butyl alcohol (TBA) plant in the ...

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How to Choose the Best Rapid Prototyping Method

How to Choose the Best Rapid Prototyping Method

Tuesday, 18 July 2017  |  Kate Kunkel

As new products are designed, including valve bodies and the parts that comprise the finished valve, prototypes must be created. How that is achieved ...

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Industry Headlines

Badger Alloys Joins VMA as Associate Member

3 DAYS AGO

This week the Valve Manufacturers Association (VMA) welcomes Badger Alloys as an official associate supplier member. This is VMA’s fourth new member in 2017.

Located in the heart of Milwaukee and founded in 1966, Badger Alloys offers single source capabilities for custom castings. The company pou...

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Thermodyn Joins VMA as Associate Member

3 DAYS AGO

This week the Valve Manufacturers Association (VMA) welcomes Thermodyn Corporation as an official associate supplier member. This is VMA's third new member in 2017.

In 1979, Thermodyn began business with the dual purpose of selling A.W. Chesterton products and manufacturing high-temperature elastomers ...

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LyondellBasell to Build the World's Largest PO/TBA Plant

13 HOURS AGO

LyondellBasell has made the final investment decision to build the world's largest propylene oxide (PO) and tertiary butyl alcohol (TBA) plant in the Houston area. The project is estimated to cost approximately $2.4 billion, representing the single-largest capital investment in the company's history...

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EPA Selects Projects for Water Infrastructure Loans

1 DAY AGO

The EPA is inviting 12 projects in nine states to apply for Water Infrastructure Finance and Innovation Act (WIFIA) loans. These potential applicants were selected from a group of projects that represent large and small communities from across the U.S. that submitted letters of interest to EPA in Ap...

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Leading Economic Indicators Increased in June

13 HOURS AGO

The Conference Board Leading Economic Index (LEI) for the U.S. increased 0.6% in June to 127.8 (2010 = 100), following a 0.2% increase in May, and a 0.2% increase in April.

“The U.S. LEI rose sharply in June, pointing to continued growth in the U.S. economy and perhaps even a moderate improvement...

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U.S. Jobless Claims Fall to Near Five-Month Low

1 DAY AGO

In the week ending July 15, the advance figure for seasonally adjusted initial claims was 233,000, a decrease of 15,000 from the previous week's revised level. The previous week's level was revised up by 1,000 from 247,000 to 248,000. The 4-week moving average was 243,750, a decrease of 2,250 from t...

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Austenitic Stainless Steels

materials_q_and_a_graphicQ: Do austenitic stainless steels require impact testing when used in low-temperature applications?

 

A: Austenitic materials are generally recognized for their lack of ductile-to-brittle transition behavior. In other words, they do not generally display a reduction in impact energy as the test temperature is reduced. By contrast, ferritic and martensitic materials-such as the carbon steels, alloy steels, and 400-series stainless steels-exhibit reduced toughness as test temperature is reduced.

 

Wrought vs. Cast
The wrought forms of the austenitic stainless steels used for pressure-retaining components (forgings, plate, pipe, etc.) are typically supplied in the solution-annealed condition with a fully austenitic structure. These materials suffer essentially no reduction in toughness even at cryogenic temperatures. For example, notched Charpy specimens of Types 304 and 316 stainless steel in the solution-annealed condition will routinely stop the hammer on smaller Charpy machines. On larger machines, the impact will bend the specimens far enough that they pass between the specimen supports and are thrown from the tester, but they will not crack or break.

Cast versions of the austenitic stainless steels are slightly different. The cast versions have slightly modified chemistries that are balanced to produce a microstructure containing some ferrite. This is done to prevent hot cracking during solidification and cooling in the mold. As mentioned above, the ferrite phase will undergo a ductileto-brittle transition at low temperatures. The issue is whether the impact toughness is significantly reduced by the presence of ferrite in the amounts commonly encountered in these cast alloys.

The fifth edition of the Steel Castings Handbook (published by the Steel Founders' Society of America) includes a graph of Charpy V-notch toughness vs. temperature for CF8 (cast Type 304). The graph indicates the impact energy gradually declines from 125-215 ft-lbs [170-292 J] at ambient temperature down to 37-70 ft-lbs [50-95 J] at -325° F [-198° C]. The gradual decline indicates there is no abrupt ductile- to-brittle transition temperature, and the relatively high impact energy (when contrasted with the 15/12 ft-lbs [20/16 J] minimum requirement for ASTM A352 and ASME SA352 LCC) indicates the material is still very tough even at liquid nitrogen temperatures.

A major North American supplier of valve castings provided data on 69 heats of various CF-series castings that were impact tested at -320° F [-198° C], the temperature of liquid nitrogen. All heats exceeded 20 ft-lb [27 J] except one, and that heat, which averaged 18 ft-lbs [24 J] still exceeded the 15/12 ft-lbs [20/16 J] minimum requirement that applies to LCC.

ASTM and ASME vs. EN and PED
This would seem to indicate that impact testing of the CF-series castings is not necessary. The ASTM A351 and ASME SA351 specifications that cover these materials do not require impact testing, and in fact do not list impact testing among the supplementary requirements that are ordinarily considered suitable for use with these materials.

On the other hand, the European standard EN 10213 Part 4, which includes several grades that are similar to the CF-series cast alloys, requires impact testing at ambient temperature, with acceptance criteria ranging from 30-60 ft-lbs [40-80 J], depending on the particular alloy. When the steels are to be used at low temperatures, there are specific impact test temperatures and acceptance criteria listed for several of the alloys.

The various codes and regulations also handle this issue quite differently. For example, the ASME B31.3 Process Piping Code specifies minimum allow-able temperatures for materials in Table A-1. The minimum allowable temperature for the common austenitic materials (S30400, S31600, CF8, CF8M, etc.) ranges from -425° F [-254° C] to -325° F [-198° C], depending on the specific material grade and form. Impact testing is not required, although impact-qualified weld filler materials are required in certain cases when the materials are welded.

Yet the European Pressure Equipment Directive (PED) requires steel materials to exhibit a minimum Charpy V-notch impact energy of 20 ft-lbs [27 J] at 68° F [20° C] or at the minimum design operating temperature, whichever is lower. However, this requirement is waived for a material when there is no doubt that the essential safety requirement for toughness will be fulfilled even if the impact tests are not performed (see Guideline 7/17).

This stipulation would seem to excuse the wrought austenitic stainless steels from testing even at very low temperatures, since they do not exhibit ductile-to-brittle transition behavior. However, because of the presence of ferrite in castings, and the effect of that ferrite on the low-temperature impact properties, the acceptability of cast austenitic stainless steels for PED applications without impact testing is questionable.

It is important to note, however, that Guideline 7/17 includes this statement: "Every harmonized European steel standard has specifications for impact properties." This is true even for the wrought austenitic stainless-steel grades.

No Simple Anwser
In summary, there is no simple answer to this question. Whether impact tests are required for austenitic stainless steels for low-temperature applications depends primarily on the point of view of the customer. For the most part, the North American specifications and codes do not require impact testing of these materials for temperatures down to at least -325° F [-198° C], even in the cast form. The European specifications and codes require impact testing of all forms even when used at ambient temperature. The situation is not likely to change in the foreseeable future.

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