Effect of a modified quenching on impact toughness of 52100 bearing steels

In order to improve the uniformity of microstructures,and thus enhance the mechanical properties,espe-cially the impact toughness,of 52100 bearing steels,modified quenching heat treatments with different austenitizing processes have been designed.The results show that 740°C is the optimal holding tem-perature(approaching Ac 1=750°C)to improve the impact toughness of the steel by 37%after quench-ing and low-temperature tempering.Microstructure observations reveal that 740°C holding shortens the nucleation incubation period of austenitization phase transformation,and inhibits the overfast growth of austenite in some regions,which leads to simultaneous nucleation and growth of austenite during austen-itizing,and homogeneous dissolution of carbides in different austenite grains.The changes in microstruc-ture evolution improve the microstructure uniformity(carbide and grain size)after quenching.Uniformly distributed carbides and grain size effectively reduce stress concentration and retard crack propagation during impact loading,thus resulting in high impact toughness.

Social-ecological perspective on the suicidal behaviour factors of early adolescents in China:a network analysis

Background In early adolescence,youth are highly prone to suicidal behaviours.Identifying modifiable risk factors during this critical phase is a priority to inform effective suicide prevention strategies.Aims To explore the risk and protective factors of suicidal behaviours(ie,suicidal ideation,plans and attempts)in early adolescence in China using a social-ecological perspective.Methods Using data from the cross-sectional project‘Healthy and Risky Behaviours Among Middle School Students in Anhui Province,China',stratified random cluster sampling was used to select 5724 middle school students who had completed self-report questionnaires in November 2020.Network analysis was employed to examine the correlates of suicidal ideation,plans and attempts at four levels,namely individual(sex,academic performance,serious physical llness/disability,history of self-harm,depression,impulsivity,sleep problems,resilience),family(family economic status,relationship with mother,relationship with father,family violence,childhood abuse,parental mental illness),school(relationship with teachers,relationship with classmates,school-bullying victimisation and perpetration)and social(social support,satisfaction with society).Results In total,37.9%,19.0%and 5.5%of the students reported suicidal ideation,plans and attempts in the past 6 months,respectively.The estimated network revealed that suicidal ideation,plans and attempts were collectively associated with a history of self-harm,sleep problems,childhood abuse,school bullying and victimisation.Centrality analysis indicated that the most influential nodes in the network were history of self-harm and childhood abuse.Notably,the network also showed unique correlates of suicidal ideation(sex,weight=0.60;impulsivity,weight=0.24;family violence,weight=0.17;relationship with teachers,weight=-0.03;school-bullying perpetration,weight=0.22),suicidal plans(social support,weight=-0.15)and suicidal attempts(relationship with mother,weight=-0.10;parental mental llness,weight=0.61).Conclusions This study identified the correlates of suicidal ideation,plans and attempts,and provided practical implications for suicide prevention for young adolescents in China.Firstly,this study highlighted the importance of joint interventions across multiple departments.Secondly,the common risk factors of suicidal ideation,plans and attempts were elucidated.Thirdly,this study proposed target interventions to address the unique influencing factors of suicidal ideation,plans and attempts.