1.1 This specification covers creep-strength enhanced alloy steel castings for valves, flanges, fittings, and other pressure-containing parts intended primarily for high-temperature service (Note 1). However, they are not restricted to such applications, and the castings may be used for other applications for which the attributes of the material, as defined by this specification, are suitable 1.2 One grade of Martensitic alloy steel, Grade C91, is covered (Note 2). It is provided in two Classes, differentiated by the type of heat treatment after weld repairs. This and similar steels are characterized by a predominantly tempered Martensitic or tempered Bainitic microstructure that is stabilized by the precipitation of temper-resistant particles at various precipitate nucleation sites in the microstructure. Such steels are designed to have creep-rupture strengths significantly superior to those of alloys of nominally similar compositions, but in which the precipitates or nucleation sites are absent. Since this crucial difference cannot be revealed by room-temperature mechanical property tests, these alloys require tighter controls on manufacturing and processing. NOTE 1The grades covered by this specification represent materials that are generally suitable for assembly with other castings or wrought steel parts by fusion welding. It is not intended to imply that these grades possess equal degrees of weldability; therefore, it is the responsibility of the purchaser to establish a suitable welding technique. Since these grades possess varying degrees of suitability for high-temperature service, it is also the responsibility of the purchaser to determine which grade shall be furnished, due consideration being given to the requirements of the applicable construction codes. NOTE 2The committee formulating this specification has included one grade of material that is considered to represent a type of ferritic alloy steel suitable for valves, flanges, fittings, and other pressure-containing parts. Additional alloy steels will be considered for inclusion in this specification by the committee as the need becomes apparent. 1.3 The values stated in either SI units or inch-pound units are to be regarded separately as standard. The values stated in each system may not be exact equivalents; therefore, each system shall be used independently of the other. Combining values from the two systems may result in non-conformance with the standard.
ASME has revised Code Case 2192-7, which is for use of A217 C12a and A426 CP91 castings in ASME Section I, Power Boilers, construction. The revision to the code case involves additional requirements for inspection, heat treatment, temperature measurement during heat treatment, recording of heat treatment parameters, hardness test results, recording of repairs, and reporting. WK40321, to incorporate several of these requirements, received several negatives and comments. Many of the negatives and comments recommended removal of Grade C12A from A217 and creation of a new specification to cover creep-strength enhanced ferritic steel castings. Development of the new standard was approved by A01 during their meeting on May21, 2013. This action implements that approval. During the discussions of the negatives and comments on WK40321, Subcommittee A01.18 also approved an action to remove CP91 from A426, since no one was able to confirm that the alloy was being purchased or produced as centrifugal castings. That action, WK41829, was approved and the revised A426 specification without CP91 is in print. This action has gone through several ballots and has now evolved into its present form as a result of responses to negatives and comments, especially from the subject TG. For clarity, this action is presented as a clean copy. This new standard will address such alloys, beginning with grade C91, in castings produced by various methods, including but not limited to sand castings, investment castings, and centrifugally cast pipe. Products made to the standard will include fittings and other pipe components, pipe, valve bodies, pump bodies, and similar parts for use in boilers and pressure vessels. Key features are: two classes are created, dependent on whether weld repairs are major or minor and whether the repairs are either re-austenitized and tempered or receive a sub-critical PWHT; the PWHT minimum temperature is unchanged at 1350F [730C] because, while a reduced minimum PWHT temperature is being considered by various ASME construction codes, this change is not yet approved and if approved, will not be published until 2017; local PWHT, when allowed, is permitted only with the permission of the purchaser; and process controls to assure that weld repairs reach the minimum PWHT temperature and that the casting does not reach temperatures above the maximum PWHT temperature must be agreed upon between the purchaser and the producer.
Keywordscasting; centrifugal; ferritic; high-temperature service; investment casting; martensitic; pipe; pressure containing; steel castings
The title and scope are in draft form and are under development within this ASTM Committee.Back to Top
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