R. 0. Ritchie Associate Professor. S. Suresh Graduate Research Assistant. Department of Mechanical Engineering, Massachusetts Institute of Technology, Cambridge, Mass. 02139 C. M. Moss Graduate Research Assistant. Presently, Rocketdyne Division of Rockwell International, Canoga Park, Calif. Near-Threshold Fatigue Crack Growth in 2 1 /iCr-1Mo Pressure Vessel Steel in Air and Hydrogen As part of an ongoing program to examine subcritical flaw growth in candidate steels for proposed coal gasifier pressure vessels, an initial study is made of characteristics of ultralow growth rate fatigue crack propagation in thick-section, normalized 2'A Cr-lMo pressure vessel steel (ASTM A387, Class 2 Grade 22). Crack propagation data are generated over a wide range of growth rates, from 10~ 8 to 10~ 2 mm/cycle, for load ratios between 0.05 and 0.80 at ambient temperatures in low pressure environments of moist air, dry hydrogen gas and dry argon gas. Particular emphasis is placed on behavior at near-threshold growth rates, below 10~ 6 mm/cycle, approaching the so-called threshold stress intensity for fatigue crack growth, AK 0 . Near-threshold growth rates, in addition to showing a marked sensitivity to load ratio, are found to be significantly enhanced in gaseous hydrogen compared to air. Similar environmentally-enhanced growth is observed in argon gas. To account for such results, previous models of threshold behavior based on environmental factors (e.g., hydrogen embrittlement) are questioned, and a new approach is presented in terms of the role of oxide debris from moist environments in promoting crack closure. This oxide-induced closure model is found to be consistent with most experimental observations of near-threshold fatigue crack propagation behavior and is proposed as a mechanism for environmental effects at ultra-low growth rates. Introduction Energy requirements in this country have prompted major efforts in developing additional energy sources to oil and natural gas. One such system involves gasification of coal by reacting pulverized coal with steam at high pressures and temperatures. Processes designed to produce 250 million cubic feet of high or low BTU gas per day are being con- sidered. Such processes will require welded steel pressure vessels, as large as 60 m long, 6 m diameter, with 250-350 mm thick walls, to operate at high pressures and temperatures in the presence of erosion-producing solid particles and chemically-aggressive atmospheres containing H 2 0, H 2 , H 2 S, CH 4 etc. []]. Design of such vessels must allow for the fact that sub-critical growth of crack-like defects, which invariably are present in large-scale structures, may be vastly accelerated by the presence of such environments, particularly those containing (or producing) hydrogen. Accordingly, material requirements for the pressure vessel include homogeneous, tough, weldable steels having yield strengths in excess of 350 MPa, with good resistance to creep, fatigue and particularly environmental degradation (e.g., hydrogen embrittlement and hydrogen attack). Prime candidate materials for this ap- plication are the 214 Cr-1 Mo low alloy steels. It is the objective of one of our ongoing research programs to investigate the influence of gaseous environment on fatigue crack propagation in 2!4Cr-lMo steels, with particular emphasis on near-threshold crack growth (below 10 " 6 mm/cycle), approaching the so-called threshold stress in- tensity AK 0 , below which cracks remain dormant or propagate at experimentally undetectable rates. The present paper describes preliminary studies in thick-section nor- malized material (ASTM A387, Class 2 Grade 22), tested principally in moist air and dry hydrogen gas, to investigate effects of through-thickness microstructure, load ratio and environment on such near-threshold behavior. Since much near-threshold data in the past have been rationalized in terms of "hydrogen embrittlement" contributions to growth (to explain effects of load ratio and strength level on AK 0 , for example [2]), the precise role of gaseous hydrogen on near- threshold behavior is specifically examined. Experimental Procedures The 2'/4Cr-lMo steel used in this study was received in the form of a 184 mm thick normalized plate (Lukens heat No. 3596), conforming to the ASTM A387 Class 2 Grade 22 standard (hereafter referred to as SA387-2-22), of com- position in wt. percent shown below: Contributed by the Materials Division for publication in the JOURNAL OF ENGINEERING MATERIALS AND TECHNOLOGY. Manuscript received by the Materials Division, July 24, 1979. c 0.12 Mn 0.42 Cr 2.48 Ni 0.14 Mo 1.06 Si 0.25 S 0.020 P 0.013 Journal of Engineering Materials and Technology July 1980, Vol. 102/293 Copyright © 1980 by ASME Downloaded From: http://materialstechnology.asmedigitalcollection.asme.org/ on 01/25/2015 Terms of Use: http://asme.org/terms