Hydrogen Attack In a 3Cr-1.5Mo Steel At Elevated Temperatures

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Materials Science and Engineering, 93 (1987) 217-225

217

Hydrogen Attack in a 3Cr-1.5Mo Steel at Elevated Temperatures
S. RUOFF, D. STONE and C.-Y. LI Department of Materials Science and Engineering, Cornell University, Ithaca, NY 14853 (U.S.A.) (Received September 9, 1986; in revised form November 12, 1986)

ABSTRACT Specimens o f a 3Cr-l.5Mo steel (where the composition is inapproximate weight per cent) were exposed to hydrogen at 13.8 MPa and 600 °C to examine their susceptibility to hydrogen attack. Special attention was given to the weld and heat-affected zones and to the cold-worked base material, all o f which were expected to exhibit poorer resistance to attack than the base material. Several specimens were exposed under a simultaneously applied tensile stress which wasexpected to accelerate the rate of attack. The materials tested were found to be highly resistant to hydrogen attack. The thermodynamic and microstructural bases for this resistance are discussed. Previous findings on hydrogen attack in 2.25Cr-1Mo steels are included for comparison purposes.

The effects of thermal and mechanical history that might be experienced during fabrication and servicehave been given special attention. The materials and mechanical variables investigated included the presence of an applied tensile stress, cold working prior to exposure, base and weld material, and the heat-affected zone (HAZ) of weldments. The 3Cr-l.5Mo material investigated was found to be resistant to hydrogen attack under all the accelerated testing conditions included in this work. Thethermodynamics and microstructural basis for the improved resistance of 3Cr-l.5Mo steels over that of the 2.25Cr-lMo steels were investigated. Some previous findings on 2.25Cr-1Mo steels are included for comparison purposes.

2. EXPERIMENT 1. INTRODUCTION In steels, hydrogen attack at elevated temperatures results from the reaction 2H 2 + C --> CH4, which produces methane bubbles along the grainboundaries. If these bubbles grow to sufficiently large sizes, they become detrimental to the mechanical properties of the steel. Cr-Mo steels are used for pressure vessels and piping t h a t contain high temperature, high pressure hydrogen in coal conversion, ammonia-manufacturing and petroleum-refining plants. It has been demonstrated that the 2.25Cr-1 Mo steels (where the composition is inapproximate weight per cent) are susceptible to hydrogen attack, although less so than the plain carbon- and molybdenum-containing steels [1-4]. The present research investigates a 3Cr-l.5Mo steel as a part of a U.S. Fossil Energy Materials Program to develop steels that are more resistant to hydrogen attack. 0025-5416/87/$3.50 2.1. Materials 2.1.1. 3Cr-l.5Mo base material The test specimens weremanufactured from a steel plate 1.6 cm thick, heat treated to simulate the center of a 25.4 cm plate in the quenched-and-tempered condition. The plate was rolled down to 1.6 cm thick from a plate 10 cm thick. After rolling, the plate was held at 980 °C for 2 h and air cooled, followed by austenitizing at 955 °C for 1 h and cooling at an average rate of 17.8 °C min -1 to approximate the cooling rate of thecenter of a water-quenched plate 25.4 cm thick. The plate was tempered at 705 °C for 20 h and has an approximate room temperature tensile strength of 590 MPa (85 klbf in-2) based on hardness measurements. The composition of the plate material is given in Table 1. This heat t r e a t m e n t results in a strength level comparable with that of a quenched-and-tempered A387 (2.25Cr-1Mo) materialstudied previ© Elsevier Sequoia/Printed in The Netherlands

218 TABLE 1 Compositions (wt.%) and treatments of the base and weld metals studied
3Cr-1.5Mo a

C 0.12 S 0.002
3Cr-l.5Mo b C 0.054
S

Mn 0.84 A1 0.004

Si 0.27 Cu 0.06

Cr 2.86 Sn 0.002

Ni 0.14 Sb 0.0004

Mo 1.48 N 0.014

V

P

0.09 0 0.0013

0.011 As --

Mn 1.10
Al

Si 0.34
Cu

Cr 2.98
Sn

Ni 0.18
Sb...
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