| High strength wear-resistant steel with high strength and hardness is based on low alloy high strength steel,which has been widely applied on engineering machinery,mining machinery,vehicle and shipping industries etc.High-strength wear-resistant steel and ordinary high-strength steel are often welded together.The use of dissimilar steel welded structure not only can meet different requirements in the various working conditions,but also can reduce costs and make the best of the performance of dissimilar steel.But the welding of dissimilar high-strength steel is complicated.Especially,that of 1200MPa high strength wear-resistant steel and Q460 steel may be prone to welding cracks at low temperature.Preheating treatment is often undertaken to avoid those,while the complex process may increase the cost of production,restricting the promotion of dissimilar high steel strength welded structure in the machinery industry.Therefore,SHT1080/Q460 dissimilar high strength steel used in excavator bucket was welded by gas shielding arc welding(GMAW)without pre-heating,and the sensitivity of the crack was discussed in butt welding and lap welding.The crack propagation mechanism,the microstructure and hardness distribution of the joint were deeply analyzed.Straight Y-type groove cracking tests were conducted to research the influences of welding heat input on the cracking ratio of SHT1080/Q460 dissimilar high strength steel joint with "low intensity matching/equal matching" welding wire.Results indicated that the section crack rate was less than 10%,when the heat input varied from 14.2-18.6kJ/cm.Crack initiation and propagation paths were observed by means of optical microscope and scanning election microscopy.It had been found that all cracks stated in root fusion zone of SHT1080/Q460 butt joints.It was more easy for the crack propagating on the Q460 side,then turning to the heat affected zone after extending paralleling the fusion line.The crack propagation path on the SHT1080 steel profile crossed the fusion zone and turned into the weld along brittle phase such as proeutectoid ferrite,different from that on the Q460 side.The crack sensitivity of SHT1080 and Q460 lap joints was studied by the cracking tests of lap joint.And the results showed that there was no crack in the lap joint.The heat input should be controlled within a suitable range in the bucket production.Meanwhile,optimize the bucket structure to reduce the degree of restraint.Results of straight Y-type groove cracking tests at low temperature showed that the welding current(400A)can lead to cracks in the center of welds,although the heat input was appropriate.When welding bucket Q460 floor and SHT1080 strengthen plate at low temperature,we could get a good welded joints at the heat input range of 14.2?18.9kJ/cm,the current range of 265?296A and controlling assembly gap.The microstructures of the joint weld and fusion zone were observed.The welds of SHT1080/Q460 were composed of proeutectoid ferrite and acicular ferrite.The toughness of weld depended on the ratio of proeutectoid ferrite and the acicular ferrite,while the ratio was mainly influenced by heat input.When the heat input was less than 14.2kJ/cm,the microstructures were principally acicular ferrite,and only a small amount of proeutectoid ferrite was distributed in the austenite grain boundary.When the heat input was above 17.6kJ/cm,the weld appeared a large deal of massive proeutectoid ferrite,completely covering the austenite grain boundary,while the acicular ferrite content decreased.The difference of microstructure on both sides of joints were observed to analyze the relationship between the microstructure and the crack propagation.The fusion zone consisted of the semi-melting zone and the unmixed region.The microstructure of semi-melting zone consisted of martensite and feather-like bainite.While the unmixed area was made up proeutectoid ferrite and acicular ferrite.SHT1080 steel contains a lot of Cr element,a element for ferrite formation,as a result,ferrite was coarsening at the fusion zone of the SHT1080 side,which was the primary cause of cold cracks extending to the weld.Q460 steel contains B,Ti,V and other trace elements.On the one hand the semi-melting zone of hardening increased,resulting in forming coarse martensite and bainite.On the other hand non-metallic inclusions formed promoting the proportion of acicular ferrite in the unmixed area.So crack resistance is poor at semi-melting zone of Q460 side.The hardness of the SHT1080/Q460 joint was measured by the microhardness test.And the maximum hardness value of the SHT1080 side was 388?259HV0.5,which appeared in the fine grain area about 1mm from the fusion line.While the maximum hardness value of the Q460 side was 416-360HV0.5 in coarse crystal area,which was the maximum hardness value of the joints.In conclusion,Q460 has high hardening capacity,and it is more easy to crack for the fusion zone of Q460 side.. |
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