Study On Weldability Of 20MnMo Casting And Forging Low Alloy QT Steels

20MnMo forging and casting steel is a category of low carbon QT steels,which is widely used in heavy walled pressure vessels at moderate temperature.The equipment's safety depends on to a great extend the performance of its welded joints.There are vast amounts of research published on weldability of low—alloy steels but until now there is little reported work on that of 20MnMo steel.The main purpose of this dissertion is to investigate into the weldability of 20MnMo steel systematically and thoroughly,which not only disclose its special features,but also increase the understanding of theory on the low carbon QT steels welding.The aspects which are covered in the work decribed are as follows : 1 .cold cracking sensitivity;2.microstructure and propertities of HAZ;3.stress relief ( SR ) treatment;4.comparison between weldability of forging and casting steels.Based on the above research,the welding and treatment procedures are designed and optimized for the 20MnMo casting steel with high carbon equivalent and restraint,which could guide the its welding practice.The implant test is carried out to evluate the cold cracking susceptibility of 20MnMo steel quantitatively. The influence of chemical composition and welding procedures on the tendency to cold cracking is mainly investigated.The fluctuation of chemical composition with the extent of standard has the significant effect on the weldability of 20MnMo steel,Whatever the chemical composition fluctuated ,The resistance to cold cracking is primarily controlled by the maximum hardness (HV_max) of HAZ,with HV_max up to 350HV,there is a damatically increasing tendency to cracking.The influence of procedures factors included the preheating and post heating,arc welding process,the austenite comsumables on cold cracking are examined.The effective machanisam of these factors effecting on the resistance to cracking has been revealed by analysising the critical rupture stress,fracture morphology,microstructure and hardness in the coarse grain zone.At lower weld hydrogen levels,for steels that liable to hardening hardness can be reduced with preheating like the steels that not liable to hardening ,however, their machanisams are different.for the former,with preheating ,the cooling rate in the low-temperature range is retarded,and this gives more time for promoting the self-tempering of transformered martensite ,this is the reason that preheating could improve the microstrucres and decrease the hardness for the easily hardening steel. Relationships on self-tempering of martensite and preheating process are studied, which interpret the cause of self-temperingincreasing the the resistance to cold cracking.With respect to study of post heating,the effect of post heating is quantitatived.Its effective mechanism is discussed penetrately,indicating that the role of post heating is mainly obtained by decreasing hydrogen,instead of others.The effect of post heating on the microstructure would be overlooked.Moreever,if with the lower temperature,the second hardening would be occurred in the HAZ during the post heating ,which reducing its function.The concept of post heating equivalent parameter is introduced and expressed as following:P= Tp (C+ Intp) ,where Tp is post heating temperature (K) ,tPis holding time (hour) ,the value of C is depend on the materials,for the low-alloy steels ,C is 5.3.The proper post heating paremeter on the 20MnMo steel can be selected based on the following formula: o cr=946.12P—1727.4(MPa). The influence of arc processing,such as C02 shielded arc welding and shielded metal arc welding (SMAW) on the tendency to cold cracking is compared ,the results show that there is higher suscepbility to cold cracking by C02 arc welding than by SMAW .The main cause led to that is analysis and explained ,which correct the opinion that C02 arc welding can reduce dramatically the tendency to cold cracking due to obtaining ultra low hygrogen levels.The use of C02 arc welding and flux cored wire is increasing steadily in stuctural fabrication because of the higher deposition rate that can be obtained,the above research could help to use the C02 arc welding process properly. As far as the use of consumables is concerned.the function of austenite consumables on the cold cracking is firstly quantative,indicating that they can increase the resistance to cold cracking.however,there is signiffcant distinguish beteen the using of nickel-based consumables and that of austenite steel consumables ,the former can increase resistance to cold cracking to more great extent than the latter,which can take place of preheating process.The structure and composition of fusion zone are examined ,which explained the reason why led to failure and lowering safety fator coefficient for the austenite steel consumables.With respect to HAZ microstructure and propertities ,the influence of heat recycle on HAZ of 20MnMo forging and casting steel is studied.The effect of heat recycle on microstructures,hardness and toughness in HAZ are studied in details.In order to approaching the actual welding conditions, the influence of tn and tg/5 was distinguished.The susceptibility variables and the cause of embrittle on the 20MnMo steel at overheated zone is firstly investigated.The results showed that the toughness loss in HAZ is mainly owing to the growing coarsely of original austenite grains of ,instead the occurring of M-A constitutes.Based on the research,the parameters which including preheat temperature and heat input are optimized for decreasing overheated zone's embrittlement.The reported work on SR heat treatment of low-alloy steel is sparse.The influence of SR treatment on the HAZ's toughness is firstly studied,which disclose the extent and behaviour of SR embrittlement. The results stated that SR embrittlement is due to temper embrittlement(TE),with more alloy content,there is more toughness loss in the overheated zone.There is the most sensitivity to SR embrittlement around 580°C .With increasing the preheating temperature ,the extent of SR embrittlement could be reduced.There is little reported work on studing forging and casting steel's weldability.Theirs weldability is compared by investigating the cold cracking suscepbility and overheated zone toughness in this dissertion.The results show that casting steels has a higher tendency to cold cracking than forging steel.The main cause is related to casting steels' less tendency to hardening apart from its less sensitive to hydrogen embrittlement.There is little difference on the overheated zone's toughness between forging and casting steel at relatively lower tg/5 ,however,with retarding ts/s ,chances of overheating embrittlement seem more higher for casting steels.