Update on CSEF Steels

Transcription

1 Update on CSEF Steels Grade 91 The Reduction in Allowable Stress Values Development of Grade 91, Type 2 EHG Workshop May 2018 J. Henry ATC-CES

2 Reduction in Allowable Stress Values for 91 Not an action taken lightly Response of users has been surprisingly intense In China, anger and confusion Lack of understanding of why action was taken no problems seen after years of operation Belief that ASME has been led astray by outside parties with commercial interests Uncertainty over impact of change on operating units In US, considerable confusion at plant level over impact on future operation of plants with Grade 91 pressure parts Why was action necessary since no problems? Concern over the cost of Type 2 In Europe a more muted response - similar action taken by ECCC

3 Topics Reviewed Introductory remarks regarding CSEF Steels The process of having new materials accepted for Code use Material development Production of data Production of a material specification Assignment of allowable stress values Initial use of the material Continued testing Re-assessment Grade 91, Type 2 the technical basis Grade 91, reduction in allowable stress values Magnitude of the reduction What it means for HRSGs

4 Creep Strength Enhanced Ferritic Steels Metallurgically complex alloys that achieve improved elevated temperature strength through controlled chemical composition and heat treatment The quasi-stabilized structure produced in the CSEF steels during successful processing slowly breaks down under influence of stress at elevated temperatures

5 Creep Strength Enhanced Ferritic Steels Tests are conducted (i.e., creep and creep-rupture) to assist in predicting the rate at which the breakdown will occur over a range of stresses and temperatures For initial acceptance of material into a Code usually only a few heats of the steel are tested and the length of the tests typically is short relative to anticipated service life (by factor of 10 or more) Significant heat-to-heat variability in these steels complicates efforts to extrapolate behavior in tests to service-like conditions

6 History of Grade 91 Steel Development by Oak Ridge National Laboratory and Combustion Engineering based on earlier experience with strength-stabilized grades in US (Timken) and Europe (X20 alloy) funded by US Department of Energy By the late 1970s the target alloy composition had been identified and extensively characterized Code Case 1943 (for seamless tubing) was approved by ASME in July of 1983 for SCI applications first CSEF steel approved by ASME First major installation: replacement outlet headers installed in the late 1980s Replacement in kind i.e., thickness of Grade 91 headers was identical to the Grade 22 headers they were replacing, which was a common practice in early years of use of the alloy Increasing use in the fossil industry since that time; favored by designers for replacements and new construction, particularly where thermal cycling anticipated (e.g., HRSGs)

7 Adopting a New Code Material Stage 1 Developing the Material Producers typically develop new materials to serve a particular market and there is an obvious interest in optimizing a new material s properties relative to the demands of that market For steam-cooled pressure parts in boilers and HRSGs, a very important property is elevated temperature strength For Code acceptance the producer is incentivized to: Produce heats with most favorable chemical composition, most favorable heat treatment condition and most favorable processing route to insure uniformity in behavior At this stage production cost usually is not a major issue since only a limited number of heats will be tested and test results will establish the commercial viability of the material

8 Adopting a New Code Material Stage 2 The Data Package The producer performs certain tests in conformance with the requirements of the Code of interest (e.g., for ASME - Section II, Part D, Mandatory Appendix 5) Room temperature mechanical properties data (TS, YS, El%, RA, etc.) Elevated temperature data (creep, creep rupture) Focus of tests is base metal, but some weld metal data required

9 Adopting a New Code Material Stage 2 The Data Package The results of the tests are compiled into a Data Package that is submitted to the Code, which uses that information to quantitatively characterize the base material s typical performance through the assignment of allowable stress values over a range of temperatures For official Code purposes, the Data Package is a technical document that provides insight into representative behavior of the material From the designer s perspective allowable stress values represent an accurate portrait of the material s performance across the range of temperatures permitted

10 Adopting a New Code Material Stage 3 The Material Specification The producer generates technical requirements for the alloy to be included in a material specification adopted by a Standards-making organization, like ASTM; the specification will provide requirements for chemical composition, heat treatment, mechanical properties, etc., that any producer can use to make the alloy (as restricted by any patents) At this point, the commercial incentive for the producer is to limit guidance to other potential producers to insure superior performance of their product note the example of Grade 23 and Grade 93

11 Grade 23 For several elements critical to the enhanced creep strength of the alloy, in the original material specification broad compositional ranges were recommended that did not reflect the ranges reflected in the three required test heats Element Code Case 2199 (Orig) Test Heats min Max Carbon Boron Columbium

12 Hardenability of Grade 23 In 2008, a fabricator reported difficulties with some heats of Grade 23 tubing in obtaining the preferred fully bainitic microstructure with air cooling i. Questionable heats met all compositional requirements of recommended by the material producer ii. Subsequent investigation had indicated considerable variability in hardenability of heats, with at least one heat that could not be transformed to a fully bainitic structure even with a water quench iii. Data research disclosed that patents filed by the material inventor had requirements for minimum levels of titanium to insure protection of free boron this information was not disclosed to the committees iv. Compositional requirements modified to require minimum titanium content to protect boron

13 Heat Treatment Requirements for Grade 93 (Save 12AD) In original Code Case application, specified austenitizing requirement for the alloy was: 1050 F minimum In subsequent revision in response to concerns expressed by committee members, the requirement was changed to: C In final revision made in response to continued concerns expressed by committee members, the requirement was changed to: C In data package submitted for the alloy, all austenitizing heat treatment were carried out at 1150 C

14 Adopting a New Code Material Stage 4 Allowable Stress Values For ASME, the producer applies for a Code Case for provisional use of the material while experience is accumulated In order for the Code Case to be used, allowable stress values are developed for base metal Established procedures for data analysis Assumed that data is representative of typical material behavior Design factor applied

15 Allowable Stress Values In determining allowable stresses, the ASME Code reviews tensile, creep and stress rupture property data obtained over the temperature range of usage and applies the following criteria listed in paragraph of Section II (Materials), Part D of the ASME code. 1/3.5 of the specified minimum tensile strength at room temperature 1/3.5 of the tensile strength at elevated temperatures ⅔ of the specified minimum yield strength at room temperature ⅔ of the yield strength at elevated temperatures 100% of the average stress to produce a creep rate of 0.01% in 1,000 hr 67% of the average stress to produce rupture at the end of 100,000 hours or 80% of the minimum stress for rupture at the end of 100,000 hours as determined from the extrapolated data, whichever is lower.

16 Adopting a New Code Material Stage 5 Initial Use of the Alloy If alloy properties and particularly the allowable stress values - are attractive to designer, the alloy will be specified for use in new construction Prior to early 2000 s, initial use of new alloys often was inkind replacement for a weaker alloy e.g., Grade 91 piping was used as a direct replacement for Grade 22 piping (same OD and wall thickness) This led to over-optimistic assessments of the new material s performance Early processing problems encountered due to inexperience of fabricators and installers with more stringent processing requirements Ultimately, successful deployment and satisfactory shortterm results

17 Adopting a New Code Material Stage 6 Continued Testing If material use is widely accepted by designers, other producers offer their version of the alloy - if patent restrictions can be skirted Controls adhere to material specification requirements but not necessarily in accord with original producers best practice New producers only concerned with meeting requirements of the material specification Lean chemical composition Most cost-effective heat treatment cycle Most cost-effective production (e.g., strand cast vs ingot)

18 Adopting a New Code Material Stage 6 Continued Testing Widespread acceptance of the alloy insures that there will be continued creep and creep-rupture testing by the producer and/or by research organizations (e.g., NIMS) Longer-term data (>30K hours) becomes available and may reveal unexpected behavior, such as: Metallurgical changes that adversely affect long-term performance (e.g., Z phase in 12%Cr alloys) Systematic changes in alloy behavior based on stress level (e.g., Kimura region-splitting criterion) Greater heat-to-heat variability, reflecting variations in production methods (inhomogeneity in structure for tubing per NIMS investigation)

19 Effect of Test Stress on the Prediction of Material Behavior From Kimura

20 Adopting a New Code Material Stage 7 Re-Assessment If Code organizations are doing their job and monitoring performance trends reflected in new, longer-term data, then for all the reasons cited above re-assessment of data is likely to lead to reduction in allowable stress values This has happened for Grade 122, Grade 92 and, most recently, for Grade 91

21 Modifications to Allowable Stress Values for Grades 92 and 122 (2007) MPa Temperature - F Grade 92 Original Grade 92 Revised Grade 122 Original Grade 122 Revised

22 Revised Allowable Stress Values for Grade 91 (to be published in 2019) MPa Temperature - C ASME 2017 t 75mm ASME 2017 t > 75mm Revised 91 Type Revised 91 Type

23 Section I Concerns In early 2014, BPV I Chairman (David Berger) wrote a letter to BPV II chairman expressing concern about use of CSEF steels in Code construction due to inability to detect damage until late in life of component As a steward of operating plants, I need materials to behave in ways that give me a fighting chance to protect my people. Specifically, there must be a fair margin of operating time between threshold of damage detection and final failure for credible damage mechanisms whenever possible. The prospect of sudden failures with little or no warning is something that should give all persons charged with the safety of pressure equipment cause to lose sleep we need to find a way to obtain information on how the material fails at elevated temperature, not merely when it fails. In other words we need some understanding of the damage accumulation dynamics and the margin between threshold of damage detection and failure.

24 EPRI Evaluation of Failed Grade 91 Header Testing of weldments from a failed header provided indications of abnormally weak heats of material

25 Variations in Parent Metal Behavior Significant differences in both strength and rupture ductility observed in different heats of Grade 91, all of which had been fabricated in the same manner and operated under exactly the same conditions

26 Influence of Chemical Composition A comparison of good heats i.e., higher strength and reasonably good rupture ductility with less favorable heats showed an apparent compositional influence

27 Compositional Issues Grade 91 Lower rupture ductility and lower rupture life are accompanied by a much higher density of creep cavities in the rupture zone just as was the case with the failed Eddystone austenitic piping

28 Compositional Issues Grade 91 Compositional requirements adopted by ASTM in mid-1980s departed in significant ways from requirements recommended by CE/ORNL

29 Existing vs Code Case 2864 Chemical Requirements Weight % New Existing C Same Mn P max Same S max Si Cr Same Mo Same W max Ni max V Same Nb Same N Cu max Al max B max Ti max Same Z max Same As max Sn max Sb max N/Al min

30 Revised Allowable Stress Values for Grade 91 (to be published in 2019) MPa Temperature - C ASME 2017 t 75mm ASME 2017 t > 75mm Revised 91 Type Revised 91 Type

31 Issues to Be Considered (Positive) Creep and creep-rupture tests conducted in air and often with relatively small diameter specimens oxidation effects lead to under-estimation of material strength Design factors applied usually are very conservative For ASME, average component life based on creep exhaustion ~ 10 6 hours

32 Issues to Be Considered (Negative) Failures in Grade 91 components seldom occur in base metal most failures in welds, where inherent weaknesses may not be factored into design Inspection techniques may not provide early warning of concerns Becoming increasingly clear that consideration of strength alone is not adequate for safe operation of components fabricated from these alloys the ability of the material to tolerate damage (e.g., reduced susceptibility to cavitation) is at least as important as strength No Codes address this issue adequately

33 What Does It Mean for HRSGs? For new HRSGs, designers will look to stronger CSEF grades, particularly Grade 92 or Grade 93, or to austenitics - not necessarily a good outcome Questions remain over the low rupture ductility of Grade 92 Very little experience with Grade 93, even in Japan

34 What Does It Mean for HRSGs? For existing fleet, the reduction in allowable stress values does not mean operating equipment suddenly is at risk If there are problems, probably not related to the allowable stress value reduction Poor design Poor control of operating conditions Poor QC during fabrication and installation

35 Appropriate Response to Lowering of ASVs Staged assessment of plant condition Review design limits to determine if change in stress values raises concerns for operating life If there are concerns based on review of design limits, compare design limits to actual operating conditions to determine if concerns persist If concerns persist, initiate comprehensive condition assessment program, including stress analysis and NDE, to identify areas of greatest concern If results indicate problems assuming minimum material properties, carry out destructive testing to characterize the heat-specific strength in problem areas as a basis for establishing the need for any repair/replacement actions that may be appropriate and to determine rational reinspection intervals

36 Thank you for your attention - Questions?