Effect Of Tungsten And Cobalt On The Structural Stability And Mechanical Properties Of Austenitic Heat-Resistant Steel

Energy shortage and carbon dioxide pollution have always been concerns of countries all over the world,and the development of super(super)critical thermal power units with higher efficiency and larger capacity has become the general trend.The material of the critical parts of the ultra-supercritical unit boiler is usually austenitic heat-resistant stainless steel.Currently commonly used Super304H(18Cr-8Ni series)and HR3C(20Cr-25 Ni series)When the boiler steam temperature parameter reaches 620 ?,Super304 H is resistant to high temperature oxidation Performance is difficult to meet the demand,HR3 C is difficult to meet the strength requirements.China is carrying out technical research on steel for ultra-supercritical thermal power stations above 650 ?.Sanicro25 not only has excellent resistance to steam oxidation corrosion at650 ?,but its durability is also 45% higher than HR3 C.It is expected to be a candidate material for the key parts of 650 ? ultra-supercritical thermal power generating units.Sanicro25 optimizes the Cr-Ni content on the basis of HR3 C,and adds 3W-1.5Co to strengthen its solid solution strengthening.The W and Co elements are two important alloying elements in austenitic heat-resistant steel,which have a significant influence on the performance of Sanicro25.This paper takes Sanicro25 as the research object,adjusts the content of W and Co in the composition,and designs and refines three austenitic stainless steels with different compositions.After holding solution heat treatment at1220 ? for 45 minutes,the three austenitic stainless steels were aged at 700 ?for 72 h,240 h,1000 h,3000 h.The microstructure after aging was characterized by using OM,SEM-EDS,XRD and EBSD.Observe and study the influence of W and Co elements on the thermal stability of austenitic heat-resistant steel structure;meanwhile,the test steel after aging is subjected to room temperature stretching to explore the aging of W and Co elements at service temperature Precipitation behavior of the second phase and its effect on the mechanical properties of austenitic heat-resistant steel.The main conclusions are as follows:(1)The austenitic stainless steel is austenite after solid solution treatment,and there are more twins.At the same time,a small amount of fine Nb C particles are distributed on the austenite matrix.The microstructure of hot rolling after solid solution treatment is obviously improved,and the precipitation phase quantity and dispersion distribution after 1220 ? heat preservation for 45 min is less,which is suitable for solid solution treatment temperature.The alloy steels after solution treatment were aged at 72 h,240 h,1000 h,3000 h,and700 ?,respectively.The results show that with the aging time of the three alloy steels,the precipitated phase begins to gradually move into the grain along the grain boundary.Diffusion,and the precipitated particles also tend to coarsen.Compared with the alloy without Co element,after adding Co element,it is obvious that the precipitation rate of the precipitated phase is slowed down and the number of twins is increased.At the same time,the precipitation phase of the2.5 wt%W alloy is smaller than that of the 1.5 wt%W alloy,and the 2.5 wt%W alloy begins to precipitate a fine Laves phase at the grain boundaries and within the grain after aging for 1000 hours,which plays a second phase strengthening role.(2)After high temperature stretching of the solution-treated structure at650 ?,700 ?,and 750 ?,it was found that the high temperature tensile strength of the three alloy steels decreased with increasing temperature.After adding 5 wt%Co,The tensile strength and yield strength of alloy steel are higher than those of 0Co alloy steel,and the tensile strength and yield strength of 2.5wt%W alloy steel are also higher than those of 1.5 wt%W alloy steel.This shows that the addition of W and Co has a significant effect on improving the mechanical properties of austenitic heat-resistant steel.The alloy steels under different aging times were stretched at room temperature,and the results obtained showed that the precipitation phases of the three alloy steels began to increase with aging time due to aging hardening at the early stage of aging.The tensile strength and yield strength were at The highest was reached at 240 h.With the further extension of the aging time,the precipitated phases were densely diffused into the crystal,and the tensile strength and yield strength decreased.Compared with the addition of Co alloy and the absence of Co alloy,it is found that the addition of Co element can significantly improve the strength and elongation of the alloy.The strength of the 2.5 wt%W alloy does not decrease significantly after long-term aging,and the toughness can still maintain a good state.