What are the main differences between CrMo steel and 304 stainless steel in performance and application?
CrMo steel has obvious advantages in strength. The yield strength of 15CrMo (≥295MPa) is about 45% higher than that of 304 stainless steel (≥205MPa), which is more suitable for pressure-bearing parts. However, the corrosion resistance of stainless steel far exceeds that of CrMo steel, especially in chloride ion environments. In terms of cost, the price of 304 stainless steel is 3-4 times that of CrMo steel. In terms of application selection, stainless steel must be used in the food and pharmaceutical industry, while CrMo steel is preferred for high-temperature and high-pressure pipelines. Under working conditions above 550℃, stainless steel will undergo σ phase embrittlement, while CrMo steel still maintains good performance.
How big is the gap in high-temperature performance between CrMo steel and P91/P92 steel?
The allowable stress of P91 (9Cr-1Mo-V) steel at 600℃ is 2.5 times that of 15CrMo, and its creep resistance is significantly improved. P92 steel increases the upper temperature limit to 630℃ by adding 1.8% tungsten. However, the welding process of high chromium steel is more complicated, and preheating (300-350℃) and PWHT (760-780℃) need to be strictly controlled. In terms of cost, the price of P91 steel pipe is 60%-80% higher than that of 15CrMo. In ultra-supercritical units, P91/P92 has gradually replaced traditional CrMo steel.
How is the economic comparison between CrMo steel and carbon steel (such as Q235)?
The material cost of 15CrMo steel is 40%-50% higher than that of Q235 carbon steel, but because of its higher strength, it can reduce the wall thickness by 20%-30%, and the actual amount can be saved by 15%-20%. Under high temperature conditions, the service life of CrMo steel is 3-5 times that of carbon steel, and the comprehensive cost is lower. However, for non-critical pipelines with a design temperature of <300℃, carbon steel is still a more economical choice. Considering the cost of the entire life cycle, it is more cost-effective to use CrMo steel for high temperature and high pressure equipment.
How to divide the applicable boundaries between CrMo steel and nickel-based alloys (such as Inconel 625)?
When the temperature exceeds 600℃ or the medium contains high concentrations of chlorides, nickel-based alloys should be used instead of CrMo steel. The life of Inconel 625 in extreme corrosive environments is more than 10 times that of CrMo steel, but the price is as high as 8-10 times. For sulfur-containing oil and gas environments with temperatures of 450-600℃, composite pipes lined with nickel-based alloys in CrMo steel are the most cost-effective choice. Nickel-based alloys must be used in special occasions such as the primary circuit of nuclear power plants to ensure safety.
What new materials may replace traditional CrMo steel in the future?
New ferritic heat-resistant steels (such as NF709 and SAVE25) can replace part of CrMo steel at 650℃ by adding elements such as copper and cobalt. ODS (oxide dispersion strengthened) steel has excellent radiation resistance and is a candidate material for the next generation of nuclear reactors. Metal-based composite materials (such as CrMo steel + SiC fiber) can improve both strength and corrosion resistance. However, in the next 20 years, traditional CrMo steel will still maintain its mainstream position in the medium temperature field, and its market share is expected to remain above 60%.
