SA335-P91 VS SA335-P22

With the development of the power industry, the capacity of thermal power plants is getting larger and larger, and new medium and high alloy steels are emerging, such as martensitic heat-resistant steel ASME SA335-P91 and pearlitic heat-resistant steel ASME SA335-P22.

Comparison of chemical composition and mechanical properties of SA335-P91 and SA335-P22 steels:

Chemical composition(%):

Grade	C	Si	Mn	Cr	Mo
SA335-P91	0.08～0.13	0.05～0.20	0.30～0.60	8.00～9.50	0.85
SA335-P22	≤0.15	≤0.50	0.30～0.60	1.90～2.60	0.78～1.13

Mechanical properties:

Grade	Yield Strength(min) (Mpa)	Tensile Strength(min) (Mpa)	Elongation(min)(%)	Hardness(max)(HB)
SA335-P91	415	585	20	250
SA335-P22	205	415	30	163

SA335-P22 is a pearlitic heat-resistant steel with high heat strength, thermal stability and corrosion resistance, and the martensite transformation temperature is 430°C-450°C. SA335-P91 is a martensitic heat-resistant steel whose structure is at the boundary of M-F, which has higher heat strength, thermal stability and corrosion resistance than the former.

SA335-P91+SA335-P22 dissimilar steel welding problems:

1. The higher the alloy content, the easier it is to form hardened structure.
The Cr, Mo and V in the steel make the C curve strongly shift to the right, increasing the hardenability of the steel. During the post-weld cooling process, the martensite brittle structure is generated in the weld zone and the heat affected zone, and the high alloy side The cold cracking tendency of the weld and the near seam area is greater.

2. Carbon migration under long-term high temperature
In the joints operating in the heat-affected zone and high temperature, due to the large difference in chemical composition of the fusion zone, both sides of the fusion zone are prone to carbonation and decarburization. The high alloy side is heavily carbonized to produce coarse carbides, and the low alloy side is severely desorbed. The carbon forms a wide, low strength F-belt that forms a low strength brittle joint.

3. Heat affected zone softening
During the welding process, the base metal is heated to a tempering zone near Acl where austenite (A) decomposition products, polymerized carbides and a large amount of ferrite from martensite to pearlite appear extremely close. The annealed state of steel is called the softening zone. Under the long-term high temperature load, the permanent strength and plasticity of this area are greatly reduced, and the depth of the softened layer is proportional to the time of staying near Acl.

Methods of solution:

1. Prevent cold cracks
Pre-weld preheating and control of interlayer temperature and reduction of line energy can reduce and reduce welding thermal stress and post-weld residual stress; keep the workpiece temperature higher than martensite transformation temperature during welding, and avoid austenite to horse The transformation of the body to prevent the formation of hardened structures; the selection of low-hydrogen electrodes and reasonable welding processes.

2. Overcome, reduce carbon migration and reduce softening depth

The electrode with chemical composition between the two or the transition zone of chemical composition in the weld metal is selected to control the heat treatment process parameters to prevent the carbon migration from increasing and the softening band widening due to excessive temperature.

Applicable scenarios:

‌SA335-P91 steel‌: Mainly used for heating surface pipes, economizers, superheaters, reheaters and other components of high-pressure boilers. Due to its excellent high-temperature performance and toughness, P91 steel has been widely used in supercritical and ultra-supercritical power plant boilers. P91 steel is particularly suitable for manufacturing boiler headers and main steam pipes with metal wall temperatures ≤600℃.

‌SA335-P22 steel‌: Mainly used for heating surface pipes of low and medium pressure boilers and some low-pressure application scenarios. Due to its good process performance and excellent welding performance, P22 steel was widely used in subcritical units in the past, but its high-temperature strength is low and is suitable for applications with lower temperatures.

Differences between P91 and P22 seamless steel pipes:

1. Performance difference

Strength: The room temperature yield limit of P91 steel is twice that of P22 steel, and the tensile strength is 41% higher than that of P22 steel. Below 650℃, the allowable stress of P91 steel is higher than that of P22 steel. At 550℃ and 105h, the creep strength of P91 steel is almost twice that of P22 steel.

Antioxidation: P91 steel contains higher chromium elements, etc., and has better oxidation resistance than P22 steel, and can form a denser and firmer oxide film at high temperature.

Welding performance: Both have good welding performance, but P91 steel is sensitive to weld IV type cracks. More measures need to be taken to ensure quality during welding, such as controlling heat treatment temperature, and the welding process is relatively complicated.

2. Difference in use

P91 seamless steel pipe: Mainly used for high-temperature superheaters and reheaters of subcritical and supercritical boilers with wall temperatures ≤625℃, as well as high-temperature headers and steam pipes with wall temperatures ≤600℃. It can also be used for nuclear power heat exchangers and petroleum cracking unit furnace tubes, etc., suitable for high temperature and high pressure environments.

P22 seamless steel pipe: used for boilers, heat exchangers and fluid delivery pipelines in petroleum refineries, boilers, superheaters, heat exchangers, condensers, catalytic tubes, etc. in chemical enterprises, as well as some pipeline systems that require corrosion resistance. The working temperature is generally lower than that of P91 steel pipes, and the maximum working temperature is about 580℃ - 590℃.

Relationship between SA335-P91 and T22:

SA335-P91 is the ASME SA-335/SA-335M standard steel grade (seamless ferritic alloy steel pipe for high temperature). P91 steel is a new type of heat-resistant steel made by adding V, Nb, N and other elements to P9 (9Cr-1Mo) steel. It can be used to manufacture boiler headers, main steam pipes, etc. with metal wall temperatures ≤ 600℃.

T22 belongs to the ASTM SA213 standard, which is specifically used for seamless ferrite and austenitic alloy steel pipes for boilers, superheaters and heat exchangers. The chemical composition and performance of T22 and P22 are similar. T22 is mainly used for the heating surface of the boiler, which needs to be in direct contact with the flue gas, so the material is required to have good high temperature resistance and corrosion resistance. P22 is used to connect pipes.

Equivalent to the German standard 10CrMo910 material, T22 material combines the advantages of ferrite and austenite, has high strength and good toughness, as well as excellent high temperature resistance and corrosion resistance. T22 corresponds to 12Cr2MoG steel grade (GB5310 standard) in China.

Read more: ASTM A335/A335M Seamless Ferritic Alloy Steel Pipes