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ASME reviews test requirements for pipes, fittings, flanges
Standards organization draws line on impact testing exemptions
- By Naddir M. Patel, P. Eng.
- May 26, 2020
- Article
- Metals/Materials
Editor’s Note: This article is a followup to “Causes and cures for ductile spool component failures” by Naddir M. Patel from April 2019.
Traditional alloys have standard roles in metal fabrication, many of which are familiar to nearly everyone involved in the industry. For example, mild steels such as SAE 1008 and 1010 generally are structural steels, preferred for fabrication applications that by and large don’t involve humid or corrosive environments; carbon steels such as SA106 grade B piping and SA105 flanges are suitable for pressure piping fabrication for applications as cold as -20 degrees F (-29 degrees C); and low-temperature carbon steels such as SA 333, SA 350LF2, and SA 420WPBL6 are mandated in some locations for pressure vessel and pressure piping applications for temperatures as cold as -49 degrees F (-45 degrees C). Other conventional alloys and common uses for them include SAE 316, one of a few austenitic stainless steels used for medical applications and in industries that deal with severe corrosion, such as refining and petrochemical processing; chrome-moly steel, favored for high-temperature corrosive service and in many high-performance automobile applications; and aluminum and titanium, which have high strength-to-weight ratios and high corrosion resistance, making them especially fitting for aerospace, refinery, and chemical processing applications.
So it is with carbon steel alloys used in flanges, fittings, and pipe for chemical processing plants and refineries. The materials used in these applications must be ductile enough to resist brittle fracture.
Standards organizations such as the American Society of Manufacturing Engineers (ASME) and ASTM Intl. (formerly known as the American Society of Testing and Materials) provide guidance in such matters. Two relevant industry codes, ASME VIII Div. I and ASME B31.3, consider carbon steels—any ferrous material having 0.29 to 0.54% carbon and 0.60 to 1.65% manganese—to be ductile enough for service in hot climates, mild zones, and areas in which the temperature gets as low as -20 degrees F.
Until recently, neither ASME nor ASTM required impact testing to confirm ductile behavior for many carbon steel items used at these temperatures. The decisions to exempt certain products were based on the historic properties of the materials. For example, carbon steel products such as A105 flanges, A234 WPB fittings, and A106 grade B carbon steel pipe with a wall thickness of ½ in. and less, when used at a minimum design metal temperature (MDMT) of -20 degrees F, have been exempt from impact testing because of their traditional role in such applications.
However, historic acceptance and traditional applications don’t necessarily hold up forever. Some materials that fall under Curve B of the 2017 revision of ASME UCS-66 (see Figure 1) have a recent documented history of failures due to brittle fracture at temperatures higher than -20 degrees F, and in many cases at warm temperatures. Therefore, they are considered to be at risk of brittle fracture at ambient temperatures, mainly during start up, shut down, hydrostatic testing, and rapid depressurization (autorefrigeration). As discussed in a previous article in The Tube & Pipe Journal, “Causes and cures for ductile spool component failures,” two reasons for brittle failure have been identified: chemistry and heat-treating practices.
A Means of Mitigation
In 2019 ASME responded to this phenomenon by reassigning A105 forged flanges and A216 grade WCB castings having wall thickness less than 2 in. to Curve A. This means that Curve A now applies to:
- All carbon and all low-alloy steel plate, structurals, and bars not listed in Curves B, C, and D.
- SA217 grade WC6 if normalized and tempered or water-quenched and tempered.
A/SA105 forged flanges supplied in the as-forged condition.
Reassigning these materials to Curve A increases the allowable MDMT by 38 degrees F (21 degrees C) without requiring impact testing. This means that a design that was satisfactory for service to -20 degrees F in 2017 must be rated to 18 degrees F (-8 degrees C) in 2019 if material selection specifications remain the same. Overlooking this code requirement, either for flanges or valve bodies, risks having to either scrap or re-rate a manufactured pressure vessel or spool.
ASME Curve B materials in the 2019 requirements apply to the items in the following list:
1. Items as described in various ASME standards:
- A/SA105 forged flanges produced to fine grain practice and normalized, normalized and tempered, or quenched and tempered after forging
- SA216 grade WCA if normalized and tempered or waterquenched and tempered
- SA216 grades WCB and WCC for thicknesses not exceeding 2 in. if produced to fine grain practice and water quenched and tempered
- SA217 grade WC9 if normalized and tempered
- SA285 grades A and B
- SA299
- SA414 grade A
- SA515 grade 60
- SA516 grades 65 and 70 if not normalized
- SA-612 if not normalized
- SA662 grade B if not normalized
- SA/EN 10028-2 grades P235GH, P265GH, P295GH, and P355GH as rolled
- SA/AS 1548 grades PT430NR and PT460NR
- SA/GB 713 grade Q345R if not normalized
2. Except for cast steels, all materials of Curve A, if produced to fine grain practice and normalized, that are not listed in Curves C and D
3. All pipe, fittings, forgings and tubing not listed for Curves C and D
4. Parts permitted under UG-11 even when fabricated from plate that otherwise would be assigned to a different curve
All materials listed in 1 and 3 for Curve B, if produced to fine grain practice and normalized, normalized and tempered, or liquid quenched and tempered as permitted in the material specification and not listed for Curve D, may be considered Curve C.
To summarize, a vessel designed to the 2017 ASME code using either an A105 forging in an as-forged condition or a A216 WCB casting does not meet the 2019 requirement unless it is upgraded to the requirements for Curve B or Curve C materials. That is to say, A105 must be normalized, fully killed, and produced to fine grain practice, and A216 must be water-quenched, tempered, and produced to fine grain practice.
Procurement in Accordance with the 2019 ASME Update
To comply with the 2019 update, to secure materials under Curve B and Curve C, and to meet the -20 degrees F MDMT requirement, it is critical to procure only products that comply with the following descriptions or definitions:- A105 forgings that are normalized, normalized and tempered, or quenched and tempered after forging and produced to fine grain practice
- A216 castings grades WCB and WCC that are water quenched and tempered and produced to fine grain practice
Quenching is followed by tempering to relieve internal stress, reduce hardness, and thereby increase toughness.
To comply with these requirements, all purchase orders dated after November 2019 should call out the following specific requirements:
- A105: fully killed (ideally aluminum-killed) condition, normalized per section 7.1.4 of ASTM A961, and manufactured to fine grain practice per ASTM A941 or per section 8.3 of ASME SA-20—that is, fully killed steel with grain size finer than 5.
- A216 grades WCB and WCC: Sup- plementary requirements S15, S51.1, S51.3, and S52.1.
For critical applications, in both the above cases, a grain size finer than 7 is recommended.
Because the deliberate addition of microalloys has been used by some mills to reduce heat-treatment temperature and time, a practice known to cause brittle fracture in carbon steels, another recommendation concerns specifying the following trace element control limits:
- The ratio of manganese to carbon should be greater than 5 to 1
- Niobium, titanium, vanadium < 0.02% each
- Niobium and vanadium, combined < 0.03%
- Boron < 5 parts per million
A fool-proofing method would be for material test certificates (MTRs) conforming to EN10204 section 3.1 to have heat numbers traceable to the impact-tested MTR issued by the steel mill.
The Beginning of the End
This isn’t a new problem, not by a long shot. It’s hard to say when it started or how deep and wide the problem runs, but the 2019 update to ASME UCS-66 isn’t the end of it. If anything, it’s just the beginning of the end.
Beyond ASME’s update, analyses have been done, papers have been written, lawsuits have been filed, and—as reported at Pipeline & Gas Journal’s website on Oct. 9, 2019—one lawsuit has been settled in favor of the plaintiffs (“Weldbend, Boltex Win Lawsuit Against Ulma Forja”).
The scope of the problem—the number and size of the steel mills that engaged in such practices—will determine when industry gets to the end. More failures are likely to happen, more suits are likely to be filed, and more damages are likely to be awarded, but the end will appear only when steel mills abandon these practices.
About the Author
Naddir M. Patel, P. Eng.
Sinclair Oil Corp.
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