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An Alternative Basics Education: Valve Ed Comes to You!

An Alternative Basics Education: Valve Ed Comes to You!

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Pentair Valves & Controls Hosts Customer Appreciation Training Event

3 DAYS AGO
Pentair Valves & Controls Hosts Customer Appreciation Training Event

More than 200 representatives from Houston’s refinery, chemical and petrochemical industries joined Pentair Valves & Controls on Sept. 15 at its Pasadena, TX facility for a customer appreciation BBQ and training event . The presented seminars are a part of Valves & Controls’ Pentai...

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Mueller Water Products Recognized for Best Smart Water Solution

4 DAYS AGO

Mueller Water Products, Inc. received the 2016 Best Smart Water Solution award at the third annual Smart Water Summit in Palm Beach Gardens, FL. Mueller Water Products, who won this award for the second time, was recognized for smart technology solutions offered through its Mueller Co., Mueller System...

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Senate Overwhelmingly Passes Water Resources Development Act

3 DAYS AGO

The U.S. Senate recently passed the Water Resources Development Act (WRDA) of 2016 with a broad bipartisan vote of 95 to 3. This legislation not only authorizes critical U.S. Army Corps of Engineers projects that drive investment in navigation, flood management, and ecosystem restoration, it provides ...

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Alaska Railroad Prepares First U.S. Shipments of LNG

3 DAYS AGO

The Alaska Railroad (ARRC) will demonstrate its ability to safely transport liquefied natural gas (LNG) in intermodal LNG ISO containers from southcentral to interior Alaska during a month-long operational performance project in early fall 2016.

ARRC is the first railroad in the country to obtain permi...

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Manufacturing Growth Eases Again in September

2 DAYS AGO

At 51.4 in September, the seasonally adjusted Markit Flash U.S. Manufacturing Purchasing Managers’ Index (PMI) was down from 52.0 in August and pointed to the weakest improvement in overall business conditions since June. The latest PMI reading marked seven years of continuous growth across the ...

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Leading Economic Indicators Down Slightly in August

3 DAYS AGO

The Conference Board Leading Economic Index (LEI) for the U.S. declined 0.2% in August to 124.1, following a 0.5% increase in July, and a 0.2% increase in June.

“While the U.S. LEI declined in August, its trend still points to moderate economic growth in the months ahead,” said Ataman Ozy...

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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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