【作者】 王学；

【导师】 施雨湘；

【作者基本信息】 武汉大学 ， 液体机械及工程， 2004， 博士

【摘要】 20MnMo铸锻钢是广泛应用于中温厚壁压力容器制造的一种低碳调质钢,其焊接质量直接影响到设备的安全使用。虽然国内外对低碳调质钢焊接性进行了大量的研究,但涉及到20MnMo钢的研究工作和成果很少。全面深入的研究其焊接性,不仅可以揭示20MnMo钢焊接性的特殊性,有效解决20MnMo钢焊接生产中的具体问题,也进一步丰富和完善低碳调质钢的焊接性理论。 本文从以下四个方面对20MnMo钢的焊接性进行系统、深入、特别是定量的研究,（1）冷裂纹敏感性;（2）热影响区组织性能;（3）消除应力处理;（4）低合金铸锻件焊接性比较。综合上述四个方面的研究成果,优化大拘束度、碳当量偏高的ZG20MnMo钢焊接、热处理工艺,为焊接生产提供科学指导。 在冷裂纹敏感性研究中,采用插销试验对20MnMo钢的冷裂纹敏感性进行定量,重点研究化学成分波动、焊接工艺因素对冷裂倾向的影响。标准规定范围内的成分波动将显著影响20MnMo钢的冷裂纹敏感性,但无论成分如何波动,粗晶区的最高硬度值小于350HV是判断是否开裂的重要标志。在工艺因素影响中,主要研究了预热及后热、奥氏体焊接材料、焊接方法对冷裂倾向的影响。从临界断裂应力o_cr、断口形貌、粗晶区组织及硬度等方面揭示各工艺的主要作用机理。通过对预热作用机理的研究,得到了在低氢条件下,预热在焊接易淬硬钢中的主要作用在于改善组织、降低硬度。降低硬度是通过增加马氏体“自回火”效应来实现。并且研究了“自回火”马氏体的精细结构及与预热的关系,分析了马氏体“自回火”提高抗裂性的作用机理。通过对后热工艺的研究,对后热作用效果进行了定量,对后热作用机理进行了深入探讨,明确了后热的主要作用在于除氢,其改善组织作用可以忽略,若后热温度偏低,还可能出现粗晶区二次硬化的现象,严重削弱后热的效果。提出了后热当量参数P_后的概念及表达式P_后=T_P（C+Int_p）（对一般低合金钢,C取5.3）来表征后热工艺,给出了20MnMo钢的o_cr与P_后的函数关系。在对弧焊方法影响的研究中,比较了CO₂气体保护焊和手工电弧焊条件下冷裂纹倾向的变化,结果表明:CO₂焊条件下,冷裂倾向明显增加。并对这一结果进行了分析和解释,纠正了过去片面强调CO₂焊超低氢优点,所以冷裂纹敏感性小的错误观点。在CO₂焊和药芯焊丝逐渐占据统治地位的今天,其理论和实用价值重大。在焊接材料的影响方面,对奥氏体钢焊条和镍基焊条在提高抗裂性中的效果进行了定量,结果表明,奥氏体钢焊条的效果有限,镍基焊条更为安全,可以完全取代预热;通过对熔合区成分分析及组织精细结构的研究,揭示了奥氏体钢焊条安全系数不高,容易失效的原因。上述研究成果为大拘束度条件下,碳当量偏高的20MnMo钢防止冷裂纹工艺措施的选择提供了理论和试验依据。 在热影响区组织性能研究中,研究了热循环对20MnMo铸件和锻件过热区的影更多还原

【Abstract】 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.更多还原