Selecting the right plate steel for high-temperature pressure equipment involves a fundamental choice between carbon steel and alloy steel. The comparison between ASTM A515 Gr.65 (carbon steel) and ASTM A387 Gr.11 (chromium-molybdenum alloy steel) is critical for engineers. Their distinct compositions dictate vastly different application scenarios in petrochemical, power, and hydrogen service.
A515 Gr.65 is a medium-carbon, silicon-killed steel designed for intermediate-temperature service, typically up to approximately 800°F (425°C). It offers good strength and weldability at a lower cost but is susceptible to graphitization and significant strength loss at higher temperatures.
In contrast, A387 Gr.11 is a low-alloy steel containing approximately 1.0-1.5% Chromium and 0.45-0.65% Molybdenum. This composition provides significantly enhanced creep strength, oxidation resistance, and hydrogen attack resistance at elevated temperatures.
Key Application Scenarios Comparison
The choice between these two materials is primarily driven by operating temperature, environmental conditions, and long-term performance requirements.
For temperature service, A515 Gr.65 is cost-effective for applications up to its limit of about 425°C (800°F). For sustained operation beyond this threshold, up to approximately 595°C (1100°F), A387 Gr.11 becomes necessary due to its superior high-temperature strength and microstructural stability.
In corrosive or reactive environments, A387 Gr.11 is distinctly advantageous. Its chromium content offers good oxidation resistance by forming a protective oxide layer, while A515 Gr.65 has limited resistance. Crucially, A387 Gr.11 is the strongly preferred choice for hydrogen service, as its molybdenum and chromium content improve resistance to hydrogen embrittlement and high-temperature hydrogen attack, whereas A515 Gr.65 is generally not recommended for wet H2S or high-pressure hydrogen environments.
Typical applications reflect this divide. A515 Gr.65 is commonly specified for boiler shells, steam drums, and low-to-intermediate temperature reactors. A387 Gr.11 is the standard material for critical high-temperature and high-pressure equipment such as hydroprocessing reactors, refinery heaters, and high-temperature heat exchangers.
