高温氢腐蚀（HTHA）是一种困扰石化行业的现象，在高温氢环境（高于500°F[260°C]）中使用低合金钢。最近氢服务设备因HTHA损坏而发生的故障，使人们更加需要进一步了解低合金钢在高温氢环境中的行为。在这项研究中，一项独特的高压釜氢暴露实验评估了热处理和化学成分对碳-½钼（C-½钼）钢HTHA电阻的影响。高压釜氢气暴露实验使用了一个独特的“HTHA箱”，该箱设计用于准确模拟氢气在壁厚上的压差，就像在氢气环境中运行的管道和容器所经历的那样。 C-½Mo HTHA盒的制造也使其具有厚侧和薄侧，这允许研究具有单向氢浓度梯度的两种不同壁厚的性能。利用这种箱式技术，研究了正火和回火（N&T）以及退火和回火（A&T）条件下的两炉C-½Mo钢对HTHA的耐受性。在900°F（482°C）的温度和900 psi（6.2 MPa）的氢气压力下，将C-½Mo盒暴露1248小时。高压釜氢暴露实验的条件被选择为严格的，以加速HTHA损伤。在高压釜氢气暴露后，对四个C组的HTHA损伤程度进行评估- ½Mo HTHA盒采用以下方法:（i）测定甲烷含量，作为与氢暴露表面距离的函数，（ii）使用光学显微镜（OLM）和扫描电子显微镜（SEM）对金相试样进行表征，以及（iii）使用SEM对低温裂纹试样进行断口检查。N&T盒的性能优于A&T盒。N&T盒的贝氏体和铁素体微观结构比A&T盒的珠光体和铁素体微观结构更耐HTHA，这是因为N&T盒的贝氏体聚集体中碳化物的稳定性以及N&T盒与A&T盒相比的更高强度。 还观察到两次C-½Mo加热的HTHA电阻存在显著变化。本研究中选择的两种材料是根据它们的钼碳比（Mo/C）选择的，其中一种材料的Mo/C比高（3.80），另一种材料的Mo/C比低（2.29）。在相同的热处理条件下，Mo/C比较高的C-½Mo钢表现出优异的性能（对HTHA的抗性更强）。具有较高Mo/C比的C-½Mo钢的碳活性降低，这是因为形成了更多热力学上更稳定的碳化物，从而降低了甲烷生成的倾向。除了测定热处理和化学成分对C- ½Mo钢，通过完成详细的金相检查和实施低温开裂，对HTHA损伤的形态特征有了更深入的机械理解，以提供晶间HTHA损伤的增强视图。

High temperature hydrogen attack (HTHA) is a phenomenon which has plagued the petrochemical industry in applications where low alloy steels are employed in hot hydrogen environments (greater than 500°F [260°C]). Recent failures of equipment in hydrogen service stemming from HTHA damage have intensified the need for a further understanding of the behavior of low alloy steels in elevated temperature hydrogen environments. In this study, a unique autoclave hydrogen exposure experiment evaluated the effects of heat treatment and chemical composition on the HTHA resistance of Carbon-½ Molybdenum (C-½ Mo) steels. The autoclave hydrogen exposure experiment utilized a unique "HTHA box" that was designed to accurately simulate a hydrogen pressure differential through the wall thickness, as experienced by piping and vessels operating in hydrogen service. The C-½ Mo HTHA boxes were also fabricated such that there was a thick side and a thin side, which allowed the investigation of the performance of two different wall thicknesses with a unidirectional hydrogen concentration gradient. Using this box technique, two heats of C-½ Mo steel in the normalized and tempered (N&T) and annealed and tempered (A&T) conditions were investigated regarding their resistance to HTHA. The C-½ Mo boxes were exposed at a temperature of 900°F (482°C) and hydrogen pressure of 900 psi (6.2 MPa) for 1248 hours. The conditions of the autoclave hydrogen exposure experiment were chosen to be severe in order to expedite HTHA damage. Subsequent to autoclave hydrogen exposure, the extent of HTHA damage was evaluated in each of the four C-½ Mo HTHA boxes by the following methods:(i) determination of the methane content as a function of distance from the hydrogen exposed surface,(ii) characterization of metallography specimens using optical light microscopy (OLM) and scanning electron microscopy (SEM), and(iii) fractographic examination of cryo-cracked samples using SEM.The N&T boxes exhibited superior performance to the A&T boxes. The bainitic and ferritic microstructure of the N&T boxes was more resistant to HTHA than the pearlitic and ferritic microstructure of the A&T boxes because of the stability of the carbides in the bainite colonies of the N&T boxes as well as the higher strength of the N&T boxes in comparison to the A&T boxes. Significant variation in the HTHA resistance of the two C-½ Mo heats was also observed. The two materials selected in this study were chosen based on their molybdenum to carbon (Mo/C) ratio, such that the Mo/C ratio of one material was high (3.80) and the other was low (2.29). In the same heat treatment condition, the C-½ Mo steel with a higher Mo/C ratio exhibited superior performance (greater resistance to HTHA). C-½ Mo steels with a higher Mo/C ratio have a reduced carbon activity because of the formation of a greater fraction of thermodynamically more stable carbides, thus, reducing the propensity for methane formation. Apart from determining the effects of heat treatment and chemical composition on the HTHA resistance of C-½ Mo steels, a deeper mechanistic understanding of the morphological characteristics of HTHA damage was developed through the completion of detailed metallographic examination and the implementation of cryo-cracking to provide an enhanced view of intergranular HTHA damage.