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Influence Of Different Tempering Temperature To T91 Steel

Shannxi Shew-E Steel Pipe Co.,Ltd | Updated: Nov 28, 2018


 

Grade T91 steel has several of incomparable advantages such as excellent performance at room temperature and good creep resistance, low linear expansion coefficient, lower cost(9.5-11.5% amount of alloy) and excellent organizational stability under long term run, making it has been an irreplaceable material of boiler steel tubes used for ultra-high critical pressure power plant.

Alloyed by nearly 9 % Chromium and 1 % Molybdenum,they are also called 9 Cr 1 Mo steel based on its composition. T91 steel has strip martensite phase in hot rolling conditions which produce the higher hardness, so the tempering treatment is needed to reduce the hardness to receive good machinability. The recrystallization temperature of T91 steel is more than 790 ℃. Low temper temperature has no impact on microstructures and properties for T91 steel nearly, while higher tempering temperature can make the recrystallization organization happen to generate ferrite phase and uneven organization.

Recently, a engineer from China conduct an experiment through observation of metallography, scanning electron microscope and microhardness testing, studied the influence of structure and hardness under different tempering temperature( 300 ~ 780 ℃ ) and tempering cooling speed of T91 steel. The results showed that the hardness under 300 ~ 400 ℃ has changed little; under 500 ~ 600 ℃ is for carbide precipitation sensitive zone where a large number of carbide dispersion precipitates in the matrix, in this zone, hardness is higher and has a rising trend; when the temperature rise from 600 to 780 ℃, carbide along the grain boundary precipitation adequately and hardness reduce. Hardness reduce gradually go along with the increase of tempering cooling rate, which slow cooling through the sensitive range 500 ~ 600 ℃ at as speed of 5 ℃ / min, carbide dispersion precipitation and a higher matrix hardness; when tempering heat preservation under 780 ℃ and the faster cooling rate (50 ℃ / min) after cooling process, there appear carbide precipitation sensitive range fastly and less precipitation carbides in the matrix, the lower hardness, the better processing performance. 

  

T91 steel is used in engineer components, superheater coils, headers and steam piping with less thickness. This contributes to a higher thermal fatigue life of almost ten times than other T series steel . This allows them to increase the operating temperature to a higher level, increasing the efficiency of the power plant. T91 tubing belong to ASTM A213 which do Ferrite and Austenitic boiler superheater and heat exchanger seamless steel tube. More information about steel T91 tubing, contact us.

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