1、Characterization Study of Temper Embrittlement of Chromium-Molybdenum Steels API PUBLICATION 959 MAY 1982 American Petroleum Institute 2101 L Street, Northwest Washington, D.C. 20037 111 API PUBLa959 82 M 0732290 0087Ll94 7 m Characterization Study of Temper Embrittlement of Chromium-Molybdenum Stee
2、ls Refining Department API PUBLICATION 959 MAY 1982 American Petroleum Institute _“ API PUBL*757 82 H 0732270 0087475 7 i Nothing contained in any API publication is to be construed as granting any right, by implication or otherwise, for the manufacture, sale, or use in connection with any method, a
3、pparatus, or product covered by letters patent. This publication may be used by anyone desiring to do so. The Institute hereby expressly disclaims any liability or responsibility for loss or damage resulting from its use; for the violation of any federal, state, or municipal regulation with which an
4、 API publication many conflict; or for the infringement of any patent resulting from the use of an API publication, Every effort has been made by the Institute to assure the accuracy and reliability of the data presented. Copyright O 1982 American Petroleum Institute API PUBL*757 82 W 0732270 008749
5、6 O W PmFACE At the October 1973 API Refining meeting, it was brought to the attention of the Committee on Refinery Equipment that 2-1/4Cr-1 Mo steel, an alloy frequently used to build hydrotreater reactors, was susceptible to temper embrittlement. Reactors constructed of this alloy could poten- tia
6、lly experience a considerable loss of toughness. The embrittlement occurs partly during shop fabrication heat treatments, but more signifi- cantly, as a result of operating in the embrittling temperature range. Responding to this problem, CRE established a Task Group on Temper Embrittlement with mem
7、bers from the Subcommittees on Corrosion and on Pressure Vessels and Tanks. The task group was given two objectives: 1. To review the existing metallurgical data and research programs, and to recommend what approach should be taken to avoid embrittlement prob- lems with existing vessels. 2. To recom
8、mend what further work was required to develop steels for future pressure vessels which would be immune or less susceptible to temper embrittlement. On July 30, 1974, the API issued a cautionary letter prepared by the task group to managers of all refineries in the United States and Canada. It alert
9、ed plant operators to the concern for in-service embrittlement of low alloy chrome-moly steels, and especially of 2-1/4Cr-1 Mo in thick sections. It also suggested precautions that should be taken to minimize the probability of brittle fracture inembrittled material. Earlier, at the May 1974 Refinin
10、g meeting, CRE accepted a task group proposal that API support a multiyear program to characterize the temper embrittlement susceptibility of commercial plate, forging, and welds used in reactors. The program would investigate the extent of embrittlement that might exist in operating vessels and how
11、 best to predict the embrit- tlement that could occur. In the spring of 1975, Westinghouse Research Corporation was selected as contractor for a 5-year characterization research program as outlined by the task group, with Dr. Bevil J. Shaw as principal investigator. A total of 64 samples, mostly 2-1
12、/4Cr-1 Mo, but a few of 1-1/4Cr-1 Mo and 3Cr-1 Mo compositions, were provided by member companies, by steel suppliers, and by vessel fabricators. The 64 samples represented a large range of commercially produced steels, and included qualification welds, croppings from forgings, and nozzle and manway
13、 cutouts from plate ranging in thickness up to 6 inches. The characterization program was divided into Phases I and II. Phase I included a 64-sample study of the microstructure, grain size tensile properties, hardness, chemistry, including tramp elements, heat treatments, and toughness. Toughness da
14、ta included determination of tran- sition temperatures before and after short time step-cool embrittlement heat treatment and of the shift in transition temperature resulting from the induced embrittlement. For Phase II, 25 heats were selected for additional embrittlement study. These heats were cho
15、sen to represent both the typical and the extremes in chemistry and in susceptibility to temper embrittlement as measured by the step-cool test. The main thrust of Phase II was the characterization of these alloys for embrittlement during isothermal exposure at five temperatures from 650 F to 950 F
16、for times up to 20,000 hours. These temperatures were chosen to span the range typical of service condit ions. The report which follows is a summary covering the Westinghouse characterization work and several ancillary studies, which developed during the nearly 6 years of work by the task group and
17、Dr. Shaw. At the outset, the API program recognized that the characterization study could provide much needed. information about the temper embrittle- ment- behavior of commercial low alloy Cr-Mo steels used in reactors. It was not designed, however, to probe the temper embrittlement mechanism. Coin
18、cidentally, in late 1974, the Metals Properties Council Task Group on Tramp Elements in Pressure Vessel Steels was considering a proposal by Dr. C. J. McMahon, Jr., Department of Materials Science and Engineer- ing, University of Pennsylvania, to study the mechanism of temper em- brittlement, partic
19、ularly with respect to the effect of tramp elements. The API task group also reviewed the proposed program and conciuded that it was a highly desirable supplement to the characterization study. As a result, API entered into an agreement with the Metals Properties Council for joint API/MPC support of
20、 McMahons three-phase, 5-year mechanism study. The primary objective was to study, using laboratory heats of controlled chemistry, the individual and synergistic embrittl- ing poeency of the elements manganese, phosphorus, silicon, antimony, and tin. A number of significant conclusions on the effect
21、s of these elements were derived from this study. Some of these data have been published in an article appearing in Vol. 102, 1980, of the Transactions ASME, Journal of Engineering Materials and Technology. Other supplement- ary papers are in process of publication, and a final report is now being p
22、repared by Dr. McMahon. The API Temper Embrittlement Characterization Study benefited strongly from the support of steel manufacturers and vessel fabricators. Their representatives were active ex-officio members of the task group; their industrial backgrounds made their input to the program especial
23、ly valuable. This is an outstanding example of the benefits to be derived from close cooperation between supplier and user. As a result of the API characterization study and the jointly supported API/MPC mechanism study, along with large individual research programs by some steel suppliers and vesse
24、l fabricators, which were at least partly stimulated by the API program, there is now a much better under- standing of the causes and implications of temper embrittlement for reactors built up to about 1975. As a result of the cooperative effort, it is now possible to obtain hydrotreator reactors, b
25、oth in the United API PUBL*757 82 m 0732270 0087478 4 m States and abroad, with far greater initial toughness and with relatively little susceptibility to temper embrittlement. It is with a strong sense of achievement that the task group submits this summary report. Readers - are cautioned that the
26、conclusions it draws are applicable to materials in reactors produced up to about 1975. They are not applicable to the plate, forgings, and weld metal available today from those manufacturers utilizing the necessary controls to produce material having low temper embrittlement susceptibility. Finally
27、, it should be recognized that the user has the responsibility to specify that material with low embrittle- ment susceptibility is required, and that it cannot be assumed that all low alloy Cr-Mo material available today meets this criterion. AFI TASK GROUP ON TEMPER EMBRITTLEMENT APRIL 1982 CONTENT
28、S Section Page III PHASE I: STEP-COOLED EMBRITTLEMENT EXPERIMENTS AND MATERIAL CHARACTERIZATION o. 7 IV PHASE II: ISOTHERMAL EMBRITTLEMENT EXPERIMENTSo*o .*o14 VI1 PREDICTION OF LONG TERM ISOTHERMAL EMBRITTLEMENT. 19 X THE SEGREGATION OF ELEMENTS TO GRAIN BOUNDARIES IN Cr-Mo STEELS AFTER 20,000 EIR
29、ISOTHERMAL EMBRITTLEMENT ASSESSED BY AUGER ALYSISooo.oooo.oo.ooooooose.ooo25 XII A STUDY OF THE EFFECT OF HIGH PRESSURE HYDROGEN ON THE TEMPER EMBRITTLEMENT CHARACTERISTICS OF 40 f t-lb TTU (b) FATTE; 40 ft-lb TTE (c) AFATT (or amount of embrittlement, FATTU - FATTE) (d) AT(40) (or embrittlement jud
30、ged by shift in 40 ft-lb transition temperature). Material Kev The samples listed according to the material key are as follows: ( see page 1) 7 API PUBL*759 82 m 0732270 00875LO L W Material Key 1 PLA 2 PLA 3 PLA 1 FOR 2 FOR 3 FOR 1 SAW 2 SAW 3 SAW 2 ESW 2 SMA 3 SMA Sample Number _- 2, 50, 52, 60 1,
31、 7, 12, 13, 14, 33, 36, 45, 46, 47, 48, 54, 55, 70, 71, 72, 74, 75 64 77 9, 10, 15, 56, 57, 58, 59, 78 61 4 18, 19, 20, 21, 22, 23, 24, 25, 34, 35, 37, 38, 39, 40, 41 65 8., 16, 26, 27, 76 63, 67, 68, 69 66 Metallography Metallographic sections were prepared from each sample and etched so as to show
32、 the prior austenitic grain size and structure. Photographs were taken at X200 and X500, two typical examples are shown in Figures 1 and 2. The structure of each sample is listed in Table 1. ASTM Grain Size (Table 1) The ASTM grain size was judged by comparison with standard charts either directly o
33、n the microscope or by comparison with the photographs. The error in these determinations is about +1 in ASTM grain size (ASTM E112-74 Plate 1). 8 .“ ;“- API PUBLU757 82 m 0732270 0087511 3 m Tensile Properties (Table 1) The tensile properties were judged from one tensile. test on each sample. In th
34、e forged and plate material the tensile specimens were standard 0.505 inches diameter gauge as shown in Figure 3, type 1. The longitudinal axes of these specimens were parallel to the rolling direc- tion in the plate. Transverse tensile samples were taken from the weldments. A standard tensile could
35、 not be taken from most of the weld samples without including the heat affected zone (HAZ) within the gauge length. Since the base plate or HAZ could possibly yield prior to the weld material, it was decided to use reduced size samples which included only weld metal within the gauge length. Thus, th
36、e tensile data pro- duced for weld samples relates solely to the weld. Since the grain size is sufficiently small in each sample, grain size effects on the tensile properties should be negligible. (The effect of grain size on tensile properties is only apparent if there are less than about 5 grains
37、in a cross section). A diagram of the tensile samples used is given in Figure 3. The size of the weld tensile samples is as follows : _ Sample No. 4 8 16 34 35 37 38 39 40 41 42 43 44 67 68 69 76 - Tensile Specimen Type from Fig. 36 3 3 3 4 3 3 4 3 3 3 3 3 3 2 2 2 2 API PUBLm959 82 0732270 0087532 5
38、 Rockwell B hardness measurements were also taken on each sample (Table 1). Analytical Chemistry (Table 2) The analytical chemistry was done by the following techniques: C LECO - Combustion - IR Detection S (IR Infra-Red) O LECO - High Temperature Fusion - Thermal Conductometric N (LECO is a trade n
39、ame) Cr Mo Si Emission Spectrometer Cu Ni Mn P As Phosphor-Molybdate Technique Sn Sb The latter four tramp elements were evaluated at Westinghouse Research Center and the rest were evaluated at Lukens Steel Research. It should be noted that the tables give three significant figures, whereas in fact,
40、 the error in most is such that only two significant figures are warranted. Specifically the claimed precision is as follows (pp means parts per million): Element C S O N Cr Mo Si Precision 2 .002% k 3 ppm -I 5 ppm ? 3 ppm i- .05% i- .08% F .01% F Il 1 i Il El emen t ”. Cu Ni Mn As Sb Sn P Precision
41、 - _ i- .01% L- .01% +- .01% F 10% i- 5% 2 5% 4 5 ppm or better 10 . API PUBL*757 82 M 0732270 OB7513 7 W Also it should be noted that the analyses for samples 12, 13 and 14 showed a significant variation in Mn as follows: S amp le Mn (site B) Mn (site A) 12 37 .53 14 .39 .,59 13 .42 .67 The quoted
42、values of Mn in the tables are the average of the above estimates: Charpy Impact Data (Table 3) The data generated for this report stemmed from two sources: (1) Westinghouse Research on Samples not hitherto evaluated and (2) samples evaluated at the various suppliers laboratories. In order to reduce
43、 the total scope and expense of the program, the data supplied by various sources was accepted and the remaining data generated at Westinghouse Research Centr. Since the standard step-cooled emhrittle- ment treatment, termed the SOCAL step-cooled embrittlement, has been used in the API industry, viz
44、: 1100F - 1 hr Cool at 10F/hr 1000F - 15 hr Cool at 10“F/hr 975F - 24 hr Cool at 10“F/hr 925F - 60 hr Cool at 5“F/hr 865F - 100 hr Cool at 50“F/hr to 600F - air cool This embrittlement treatment was used throughout the series. Discussion of Phase I Data An attempt to fit an equation to the data gene
45、rated in Phase I was carried out using regression analysis techniques. Unfortunately, the distribution of elements, grain sizes and strength levels among the set was such that no specific major trend or coefficients associated with each variable could be found. At first sight this is surprising, sin
46、ce there appears to be a sufficient range of each variable within the number of samples taken. However, a statistically designed experiment for 14 elements plus grain size and strength, i.e. 16 variables would require considerably more than the 65 samples available. Moreover, the variables would be
47、arranged according to the design rather than taken at random. Consequently, it was not possible to find a fitted equation to the Phase I data for two reasons, viz. insufficient data and unfortunate organization of the variables. The correlation between the often employed “J-factor and AT (40), shown
48、 in Fig. 4 (a), is relatively poor for both plate and weld metal. The data can, however, be expressed in the form of the histo- grams and a few useful observations made. The histograms are shown in Figures 5 through 18. The upper histogram in each of these figures represents all the weld and plate a
49、nd forged samples, whereas the weld metal contribution alone is represented by the hatched areas within the total histogram. The dashed curves are an approximation to the shape of the histogram for the plate and forged samples and similarly the solid curve represents the welds. Thus, a comparison of weld and plate plus forged sample characteristics can be made within each histogram. There is a question of how well the histograms represent the Cr-Mo A387 steels. The samples were drawn from many sources, some of which were from actual pressure vessels a
