Research On Hydrogen Embrittlement Of High-strength Steel Surface Metamorphic Layer Based On Scratch Method

Manufacturing is the foundation of national economy.The development of manufacturing industry has gone through three stages: forming manufacturing,surface integrity manufacturing and anti-fatigue manufacturing.Compared to the previous two generations of manufacturing technology,anti-fatigue manufacturing can significantly reduce the weight of workpieces and increase the service life and reliability of parts.The key to anti-fatigue manufacturing technology is the construction of surface modified layer,which inevitably introduces hydrogen atoms during manufacture,processing and service.When the hydrogen concentration reaches a certain level,it can cause cracks initiate and expand rapidly in the material at low stresses,leading to serious damage and economic loss.Therefore,the surface modification layer should not only have high strength and hardness,but also have excellent hydrogen embrittlement resistance to ensure the service life of parts.Conventional hydrogen embrittlement test methods are suitable for evaluating the properties of bulk and homogeneous materials.The microstructure and properties of the surface modified layer vary in a gradient with depth.There may be limitations in evaluating the hydrogen embrittlement properties of surface modified layer using conventional test methods.In this thesis,the effect of hydrogen on fracture toughness is studied by the scratch method.A possible method for the evaluation of the hydrogen embrittlement susceptibility of surface modified layer of high-strength steels is given.The main work of this thesis are as follows:(1)The hydrogen embrittlement properties of matrix 18 CrNiMo7-6 alloy steel were investigated using the scratch method.The hardness,scratch width,scratch depth and tangential force of the material were tested before and after hydrogen charging.The results show that the surface hardness of 18 CrNiMo7-6 alloy steel increases,the scratch depth and tangential force decreases and the scratch width increases after hydrogen charging.Fracture toughness was calculated before and after hydrogen charging.The variation of the fracture toughness of the material at different diffusible hydrogen concentrations was investigated.A quantitative relationship between diffusible hydrogen concentration and fracture toughness is given.The maximum error between the fracture toughness obtained by scratching and the indentation is within 5%.(2)The hydrogen embrittlement properties of the surface modified layer of18 CrNiMo7-6 alloy steel after carburising heat treatment were investigated.The carburised 18 CrNiMo7-6 alloy steel was subjected to a layering treatment.The changes in hardness,scratch width,scratch depth and tangential force of the modified layer at different depths before and after hydrogen charging were investigated.The fracture toughness of the modified layer at different depths before and after hydrogen charging was calculated.The results show that as the concentration of diffusible hydrogen increases,the hardness and scratch width of the modified layer increase,and the scratch depth,tangential force and fracture toughness all decrease.The relationship between diffusible hydrogen concentration and fracture toughness is given for different depths of the modified layer.The fracture toughness of the surface modified layer is linearly related to the logarithm of the diffusible hydrogen concentration.The hydrogen embrittlement susceptibility decreases with the increasing depth of the modified layer.The study provides a method for evaluating the hydrogen embrittlement properties of surface modified layer,which is important for hydrogen embrittlement analysis,design and safety evaluation of mechanical components.