The Law Of Microstructure Transformation On HAZ Of High Strength Low Alloy Steels

High strength low alloy steel, which is developed on the basis of the carbon steel by adding a certain amount of alloy elements and using a variety of reinforcement mechanism and TMCP technology is advantageous in terms of high strength and high toughness performance. The kind of steel is widely used, however, it is liable to find that heat affected zone softening, embrittlement, cold crack and grains in the overheated zone growing coarse during welding. All of these limit the use of high strength low alloy steel. So researching the organization of welding heat affected zone of high strength low alloy steel has very important value and significance. The welding thermal simulation test can provide a method of magnifying and analyzing the structure of the specific region of heat affected zone.This has important research significance.In this thesis, the thermal simulation testing machine was used to go through the peak temperature of1300℃thermal single cycle simulation test of SS400, Q690D, Q960and Q1100steel respectively, and refrigerate at0.1℃/s0.5℃/s1℃/s5℃/s10℃/s30℃/s50℃/s100℃/s cooling rate, at the same time the law of the structure was analyzed by using Zeiss microscope and scanning electron microscopy, the grain size of each cooling rate and base metal were computed. The results show that for SS400steel, while the cooling rate under less than1℃/s,the pearlite and blocky ferrite are obtained, while at the range of5℃/s~30℃/s the main structure are side lath ferrite and a small amount of widmanstatten. While cooling rate is greater than30℃/s, bainite is obtained. For Q690D, while at the cooling rate of1℃/s,the pearlite and blocky ferrite are obtained.While at the range of0.5℃/s~1℃/s, granular bainite is obtained. While at the range of5℃/s~30℃/s, lath bainite is obtained.While at the range of50℃/s~100℃/s, lath martensite is btained. For Q960and Q1100, while at the range of0.1℃/s~1℃/s, granular bainite is obtained. While at the range of0.1℃/s~1℃/s, lath bainite is obtained. While at the range of0.1℃/s~1℃/s, lath martensite is btained.With the increase of total alloying elements, the structure of Q960and Q1100will not transform into blocky ferrite at low cooling rate range, but into bainite directly. blocky ferrite and pearlite of Q690D are more slight than that of SS400. Because of the increasing number of alloy elements, Bainite ferrite and martensite lath of Q960and Q1100are more tight and short.The grain sizes of every different kinds of steel are decreased with the cooling rate increasing. Since the alloying elements could pin on the grain boundary, the average grain size of Q690D, Q960and Q1100with the alloying elements is finer than that of SS400.With the increasing amount of alloying elements, especially the increasing of strong carbide forming elements, the average grain size of Q960and Q1100is finer than that of Q690D.