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Relevance in a Digital Age

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Reshoring, Robots and What Happens if We Do Bring Jobs Back?

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Industry Headlines

Brenda Combs of MRC Global Recognized

Monday, 24 April 2017  |  Chris Guy

Brenda Combs, senior director of implementation services at MRC Global, was recently honored by Supply House Times as one of 20 women to watch in the PHCP...

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Industry Headlines

Brenda Combs of MRC Global Recognized

-1 DAYS AGO

Brenda Combs, senior director of implementation services at MRC Global, was recently honored by Supply House Times as one of 20 women to watch in the PHCP-PVF supply chain industry.

Supply House Times “asked our industry partners to tell us about these difference-makers. The results are our third an...

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VanAire Names Steven Soderman CEO, Quality Manager

4 DAYS AGO

Steven Soderman has joined the leadership team of VanAire, Incorporated as CEO and quality manager. Bill VanDeVusse will continue in his role as president having previously filled the positions of both president and CEO.

Soderman has held key quality positions throughout his career most recently fillin...

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Natural Gas Inventories Above Five-Year Average

-1 DAYS AGO

Working natural gas in storage as of March 31, the traditional end of the heating season, totaled 2,051 billion cubic feet, or almost 15% above the five-year average according to EIA 's Weekly Natural Gas Storage Report . The total inventory of U.S. natural gas in storage tends to follow seasonal patter...

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Alliance Pipeline Proposing Expansion to Chicago Market

-1 DAYS AGO

Alliance Pipeline Co. has made a non-binding request for expressions of interest for additional natural gas transportation service on its system with an anticipated commencement date of Nov. 1, 2020. In response to high demand for its transportation service to the Chicago market hub , Alliance is asse...

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Are Women the Key to Closing U.S. Skills Gap?

20 MINS AGO

The Manufacturing Institute, Deloitte and APICS have just released, “Women in Manufacturing: Stepping up to make an impact that matters.” The joint study is the result of more than 600 survey responses from women professionals in the manufacturing industry, along with nearly 20 manufactu...

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Fed Beige Book Showing Continued Economic Expansion

4 DAYS AGO

Economic activity increased in each of the twelve Federal Reserve Districts between mid-February and the end of March, with the pace of expansion equally split between modest and moderate . In addition, the pickup was evident to varying degrees across economic sectors.

Manufacturing continued to expand...

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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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