Study On Material Selection And Heat Treatment Of 14.9 Grade High-strength Bolt

At present,high speed railway,wind power,nuclear power and other key fields have a large demand for 14.9 grade high-trength bolts,but the research in this field is still in its infancy at home and abroad,and there is a lack of relevant technical standards and special materials.The domestic ADF and 42CrMoVNb-NJ special steel grades have not been mass-produced,and the application effect needs to be verified in practice.In view of the above problems,according to the service conditions and failure modes of 14.9 high-strength bolts,this paper tries to select appropriate materials in the existing steel type system,and improve the strength,toughness and corrosion resistance of materials by optimizing the heat treatment and surface treatment process,so as to meet the technical requirements of 14.9 high-strength bolts.According to the service conditions and failure modes of 14.9 high-strength bolts,42CrMoVNb,40CrNiMoA,45CrNiMoVA and 34CrNiMo6 are selected as alternative materials by considering the service performance,process performance and economic benefits.The heat treatment process parameters were optimized by using single factor variables and orthogonal tests,and the influence of heat treatment process on the microstructure and properties of alternative materials was studied.The results show that:(1)The optimal heat treatment process of 42CrMoVNb is 860 ℃×20 min oil cooling +440 ℃×150 min air cooling.The hardness is 44.3 HRC,the tensile strength is 1448 MPa,and the yield strength is 1333 MPa.The yield ratio is 0.92,the elongation after fracture is 10.5%,the shrinkage of section is 50%,and the impact work at-20 ℃ is 27.1 J.(2)The optimal heat treatment process of 40 CrNiMoA is 860 ℃×15 min oil cooling +420 ℃×100 min air cooling,the hardness is 44.2HRC,the tensile strength is 1435 MPa,the yield strength is 1328 MPa,the yield ratio is 0.93,the elongation after fracture is 11.0%,and the shrinkage rate of section is 50%.The impact work at-20 ℃ is 25.9J;(3)The optimal heat treatment process of 45CrNiMoVA is 840 ℃×15 min oil cooling+440 ℃×100 min air cooling,the hardness is 44.4HRC,the tensile strength is 1445 MPa,the yield strength is 1335 MPa,the yield ratio is 0.92,and the elongation after fracture is 10.0%.The cross section shrinkage rate is 50%,and the impact work is 25.3J at-20 ℃.(4)The optimal heat treatment process of 34CrNiMo6 is 820 ℃×15 min oil cooling +440 ℃×150 min air cooling,the hardness is 44.3 HRC,the tensile strength is 1419 MPa,the yield strength is 1302 MPa,the yield ratio is 0.92,the elongation after fracture is 10.0%,and the shrinkage rate of section is 44%.The impact work at-20 ℃ is 23.5J.(5)The 42CrMoVNb structure is composed of fine carbide particles dispersed on the tempered tretinite matrix,and its mechanical properties can meet the technical requirements of 14.9 high-strength bolts.40CrNiMoA,45CrNiMoVA and 34CrNiMo6 are all tempered and distributed with fine carbide particles on the matrix.In addition to low temperature impact toughness,other mechanical properties meet the technical requirements of 14.9 high-strength bolts.42CrMoVNb,40CrNiMoA,45CrNiMoVA and 34CrNiMo6 after QPQ+ dacromet treatment and oxygen,carbon and nitrogen polyosmotic + dacromet treatment,neutral salt spray test for more than 1200 h,there is still no corrosion,protection grade it is level 10.