The structure of tempered martensite and its susceptibility to hydrogen stress cracking
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normally used for oil field applications were examined. The latter approach was taken in order to develop understanding that might be useful ultimately in developing hydrogen resistant steels with higher strengths. The correlation of microstructural changes with fracture surface morphology was an important aspect of this investigation. EXPERIMENTAL PROCEDURE The alloy series used for this investigation included SAE 4130 (1 pct Cr, 0.20 pct Mo) and two modifications containing nominally 0.035 pct Cb and 0.50 pct Mo and 0.75 pct Mo. The specific composition of each heat is given in Table I. Specimens were normalized for 30 rain at 930 ~ air-cooled, then austenitized at 900 ~ for 30 min and water-quenched. Tempering treatments of one hour duration were performed between 300 and 700 ~ in 100 ~ increments. All heat treatments were performed under an Argon stream. Tensile bars and strips for hydrogen charging were machined prior to heat treatment. The tensile bars were 8.89 cm long by 1.27 cm wide and 0.18 cm thick with a gage length 2.54 cm long by 0.64 cm wide. The samples were pulled on an lnstron Machine at a strain rate of 0.05 cm/min. Two specimens for each temper were tested. Strips for hydrogen testing were 10.16 cm by 0.48 cm by 0.15 cm in size. Two adjacent holes were drilled at a distance of 0.16 cm from the specimen edges and 3.49 cm from one end of the specimen. The holes insured that cracking occurred at the point of maximum stress in the center of the specimen. After removal of metallographic specimens, the remaining beam was polished to a 600 grit finish and placed in three point bending in a jig as described elsewhere. 7 Contact points of the beam with the jig were made of glass to decrease any galvanic or crevice effects. This left a portion of the beam
ISSN 0360-2133/80/1111-1799500.75/0 METALLURGICALTRANSACTIONSA 9 1980AMERICAN SOCIETY FOR METALS AND VOLUME 11A, NOVEMBER 1980--1799 THE METALLURGICAL SOCIETY OF AIME
Table I. Chemical Composition of Alloys
Elements, Pct Designation A B C
C
Mn
Si
Cr
Mo
Cb
Al
Ni
P
S
N
0.31 0.31 0.30
0.72 0.73 0.71
0.35 0.36 0.35
1.02 1.01 1.01
0.20 0.50 0.75
-0.032 0.038
0.070 0.078 0.070
0.06 0.06 0.06
0.017 0.016 0.017
0.015 0.015 0.016
0.017 0.018 0.018
protruding from one end of the jig, which was coupled by means of an alligator clip to a constant current source. The current density applied based on the immersed surface area of the bent beam was 5.0 m A / c m 2. The steel was made cathodic to a platinum anode. The charging solution consisted of 2N HESO4 with 1000 parts/million CS2 as a cathodic recombination poison. The solution was renewed daily on the longer term tests. This technique has been successfully used to rapidly introduce hydrogen into steel in relatively short time periods ( ~ 10 min). 8'9 The use of CS 2 poisons the hydrogen recombination reaction and increases the charging concentration of nascent hydrogen at the surface. The samples were stressed to 80 pct of the yield strength in three point bending s and charged at 25 ~ un
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