Occurrence of Dynamic Shear Bands in AISI 4340 Steel under Impact Loads

In this study, occurrence of adiabatic shear bands in AISI 4340 steel under high velocity impact loads is investigated using finite element analysis and experimental tests. The cylindrical steel specimen subjected to impact load was divided into different sections separated by nodes using finite element method in ABAQUS environment with boundary conditions specified. The material properties were assumed to be lower at the section where the adiabatic shear bands are expected to initialize. The finite element model was used to determine the maximum flow stress, the strain hardening, the thermal softening, and the critical strain for the formation of adiabatic shear bands. Experimental results show that deformed bands were formed at low strain rates and there was a minimum strain rate required for formation of transformed band in the alloy. The experimental results also show that cracks were initiated and propagated along transformed bands leading to fragmentation under the impact loading. The susceptibility of the adiabatic shear bands to cracking was markedly influenced by strain-rates. The simulation results obtained were compared with experimental results obtained for the AISI 4340 steel under high strain-rate loading in compression using split impact Hopkinson bars. A good agreement between the experimental and simulation results was obtained.

Experimental Investigation of Laser Surface Hardening of AISI 4340 Steel Using Different Laser Scanning Patterns

Laser surface transformation hardening becomes one of the most modern processes used to improve fatigue and wear properties of steel surfaces. In this process, the material properties and the heating parameters are the factors that present the most significant effects on the hardened surface attributes. The control of these factors using predictive modeling approaches to achieve desired surface properties leads to conclusive results. However, when the dimensions of the surface to be treated are larger than the cross-section of the laser beam, various laser-scanning patterns are involved. This paper presents an experimental investigation of laser surface hardening of AISI 4340 steel using different laser scanning patterns. This investigation is based on a structured experimental design using the Taguchi method and improved statistical analysis tools. Experiments are carried out using a 3 kW Nd: YAG laser source in order to evaluate the effects of the heating parameters and patterns design parameters on the physical and geometrical characteristics of the hardened surface. Laser power, scanning speed and scanning patterns (linear, sinusoidal, triangular and trochoid) are the factors used to evaluate the hardened depth and the hardened width variations and to identify the possible relationship between these factors and the hardened zone attributes. Various statistical tools such as ANOVA, correlations analysis and response surfaces are applied in order to examine the effects of the experimental factors on the hardened surface characteristics. The results reveal that the scanning patterns do not modify the nature of the laser parameters’ effects on the hardened depth and the hardened width. But they can accentuate or reduce these effects depending on the type of the considered pattern. The results show also that the sinusoidal and the triangular patterns are relevant when a maximum hardened width with an acceptable hardened depth is desired.

ANN Based Model for Estimation of Transformation Hardening of AISI 4340 Steel Plate Heat-Treated by Laser

Quality assessment and prediction becomes one of the most critical requirements for improving reliability, efficiency and safety of laser surface transformation hardening process (LSTHP). Accurate and efficient model to perform non-destructive quality estimation is an essential part of the assessment. This paper presents a structured and comprehensive approach developed to design an effective artificial neural network (ANN) based model for quality estimation and prediction in LSTHP using a commercial 3 kW Nd:Yag laser. The proposed approach examines laser hardening parameters and conditions known to have an influence on performance characteristics of hardened surface such as hardened bead width (HBW) and hardened depth (HD) and builds a quality prediction model step by step. The modeling procedure begins by examining, through a structured experimental investigations and exhaustive 3D finite element method simulation efforts, the relationships between laser hardening parameters and characteristics of hardened surface and their sensitivity to the process conditions. Using these results and various statistical tools, different quality prediction models are developed and evaluated. The results demonstrate that the ANN based assessment and prediction proposed approach can effectively lead to a consistent model able to accurately and reliably provide an appropriate prediction of hardened surface characteristics under variable hardening parameters and conditions.

热处理对双真空冶炼AISI 4340钢显微组织和拉伸性能的影响

采用光学显微镜、透射电镜和拉伸实验等研究了不同热处理状态的双真空冶炼AISI 4340钢的显微组织与拉伸性能,并与普通冶炼AISI 4340钢的强度-塑性进行对比。结果表明:淬火态双真空冶炼AISI 4340钢的微观组织主要由板条和片层孪晶马氏体组成,板条内位错密度较高;经350℃回火后有大量碳化物析出,且仍观察到片层孪晶马氏体;随着回火温度继续升高,逐渐形成回火索氏体。随着回火温度升高,双真空冶炼AISI 4340钢中片层孪晶马氏体逐渐分解消失以及板条内位错密度下降导致其强度降低,而析出碳化物的球化则是塑性提高的主要原因。与普通冶炼工艺相比,双真空冶炼显著降低了AISI 4340钢中夹杂物含量,从而提高其塑性;淬火态和350℃回火态双真空冶炼AISI 4340钢强度-塑性同步提高的一个原因是纳米级片层孪晶马氏体的存在。

高速铣削AISI 4340合金结构钢时涂层刀具磨损机理研究

针对AISI 4340合金结构钢难加工的特点,选用PVD硬质合金涂层刀具进行高速干铣削试验,选用扫描电子显微镜(SEM)观察失效刀具表面的磨损形貌特征,选用能谱分析仪(EDS)分析磨损刀具表面的元素分布及含量,揭示刀具的磨损机理。研究结果表明:刀具寿命与切削参数选取有关,随着切削速度的增加,刀具磨损加快,刀具寿命降低。硬质合金涂层刀具的主要磨损形式是前刀面磨损和后刀面磨损,前刀面磨损机理主要是粘结磨损、涂层剥落、切削刃微崩刃;后刀面磨损机理主要是磨粒磨损、粘结磨损、扩散磨损、微裂纹。

AISI4340高强钢在含氧和/或Cl^(-)高温水中的应力腐蚀行为

采用慢拉伸应力腐蚀试验与应力腐蚀裂纹扩展试验,对AISI4340钢在含饱和氧和/或0.1 mol·L^(-1)的100℃水中的应力腐蚀行为进行研究。结果表明:100℃水中存在的氧或C一均可以增大AISI4340钢的应力腐蚀倾向,但在含Cl^(-)并除氧的100℃水中的应力腐蚀倾向不显著,慢拉伸断口依旧保留部分韧性断裂特征,而在含饱和氧的高温水中AISI4340钢发生完全脆性断裂,应力腐蚀倾向显著;氧或Cl^(-)均可提高AISI4340钢在100℃水中的应力腐蚀裂纹扩展速率,氧与Cl^(-)之间存在交互作用,二者共存显著提高了应力腐蚀倾向,并导致开裂后裂纹快速扩展。

阶梯钻钻削AISI4340的轴向力和刀具温度研究

针对高强度钢AISI4340钻削过程中存在轴向力大、钻削温度高、刀具磨损严重等问题,课题组在普通麻花钻的基础上设计了一种阶梯钻,利用DEFORM-3D有限元软件对阶梯钻钻削AISI4340的过程进行仿真研究,对阶梯钻的轴向力形成理论进行分析,研究主轴转速、进给量和顶角等3个因素对轴向力和钻头最高温度的影响规律,并与普通麻花钻进行对比。结果表明:主轴转速对轴向力的影响最小,顶角对钻头最高温度的影响最小,阶梯钻在降低轴向力和减小刀具磨损方面比麻花钻更有优势;同时发现,当阶梯钻主轴转速取2 500 r·min^(-1),进给量取0.1 mm·r^(-1),第1顶角取118°时,得到的轴向力最小,钻头温度分布较为理想。研究的结果对实际钻削AISI4340参数的选择提供了一定的参考。

Numerical Investigation of Laser Surface Hardening of AISI 4340 Using 3D FEM Model for Thermal Analysis of Different Laser Scanning Patterns

Laser surface hardening is becoming one of the most successful heat treatment processes for improving wear and fatigue properties of steel parts. In this process, the heating system parameters and the material properties have important effects on the achieved hardened surface characteristics. The control of these variables using predictive modeling strategies leads to the desired surface properties without following the fastidious trial and error method. However, when the dimensions of the surface to be treated are larger than the cross section of the laser beam, various laser scanning patterns can be used. Due to their effects on the hardened surface properties, the attributes of the selected scanning patterns become significant variables in the process. This paper presents numerical and experimental investigations of four scanning patterns for laser surface hardening of AISI 4340 steel. The investigations are based on exhaustive modelling and simulation efforts carried out using a 3D finite element thermal analysis and structured experimental study according to Taguchi method. The temperature distribution and the hardness profile attributes are used to evaluate the effects of heating parameters and patterns design parameters on the hardened surface characteristics. This is very useful for integrating the scanning patterns’ features in an efficient predictive modeling approach. A structured experimental design combined to improved statistical analysis tools is used to assess the 3D model performance. The experiments are performed on a 3 kW Nd:Yag laser system. The modeling results exhibit a great agreement between the predicted and measured values for the hardened surface characteristics. The model evaluation reveals also its ability to provide not only accurate and robust predictions of the temperature distribution and the hardness profile as well an in-depth analysis of the effects of the process parameters.

AISI 4340钢高温磨损机理和微观结构研究

在高温条件下采用球面往复摩擦磨损方式,研究了温度对AISI 4340低合金高强度钢摩擦磨损机理和磨损过程中微观结构的影响。结果表明,摩擦系数随温度的升高而先增大后减小,摩损率随温度升高而先减小后增大。磨痕表面分布有形貌不同的磨削痕迹和鳞片状、颗粒状磨屑,磨痕表面及其亚表层均出现明显的塑性变形。高温条件下,环境温度越高,表面摩擦系数越大,表面粗糙度越高,晶粒细化现象越明显,硬度越高,耐磨性越好。温度较低时,磨损机制主要是磨粒磨损和黏着磨损;温度较高时为磨粒磨损、黏着磨损和疲劳剥落三者兼具的形式。

热处理对AISI4340钢风机主轴性能的影响

对AISI 4340钢风机主轴进行了热处理工艺试验,测定了热处理后主轴的力学性能。结果表明,锻造后经860℃正火和640℃回火,850℃淬火和540℃回火处理后,主轴的力学性能为：抗拉强度1250 MPa,屈服强度1 140 MPa,断后伸长率14.5%,断面收缩率55%,-40℃冲击吸收能量38.5-41.3 J,表面硬度40-42 HRC,符合技术要求。此外,采用专用夹具、垂直装炉和预冷淬火,有效减小了主轴的热处理畸变。

A Predictive Modeling Based on Regression and Artificial Neural Network Analysis of Laser Transformation Hardening for Cylindrical Steel Workpieces

Laser surface hardening is a very promising hardening process for ferrous alloys where transformations occur during cooling after laser heating in the solid state. The characteristics of the hardened surface depend on the physicochemical properties of the material as well as the heating system parameters. To exploit the benefits presented by the laser hardening process, it is necessary to develop an integrated strategy to control the process parameters in order to produce desired hardened surface attributes without being forced to use the traditional and fastidious trial and error procedures. This study presents a comprehensive modelling approach for predicting the hardened surface physical and geometrical attributes. The laser surface transformation hardening of cylindrical AISI 4340 steel workpieces is modeled using the conventional regression equation method as well as artificial neural network method. The process parameters included in the study are laser power, beam scanning speed, and the workpiece rotational speed. The upper and the lower limits for each parameter are chosen considering the start of the transformation hardening and the maximum hardened zone without surface melting. The resulting models are able to predict the depths representing the maximum hardness zone, the hardness drop zone, and the overheated zone without martensite transformation. Because of its ability to model highly nonlinear problems, the ANN based model presents the best modelling results and can predict the hardness profile with good accuracy.

Effect of heat input on microstructure and mechanical properties of dissimilar joints of AISI 316L steel and API X70 high-strength low-alloy steel

The microstructure and mechanical properties of dissimilar joints of AISI 316L austenitic stainless steel and API X70 high-strength low-alloy steel were investigated.For this purpose,gas tungsten arc welding（GTAW）was used in three different heat inputs,including 0.73,0.84,and 0.97 kJ/mm.The microstructural investigations of different zones including base metals,weld metal,heat-affected zones and interfaces were performed by optical microscopy and scanning electron microscopy.The mechanical properties were measured by microhardness,tensile and impact tests.It was found that with increasing heat input,the dendrite size and inter-dendritic spacing in the weld metal increased.Also,the amount of delta ferrite in the weld metal was reduced.Therefore,tensile strength and hardness were reduced and impact test energy was increased.The investigation of the interface between AISI 316L base metal and ER316L filler metal showed that increasing the heat input increases the size of austenite grains in the fusion boundary.A transition region was formed at the interface between API X70 steel and filler metals.

Shear Band Formation in AISI 4340 Steel Under Dynamic Impact Loads:Modeling and Experiment

In this study, the occurrence of the adiabatic shear bands in AISI 4340 steel under high velocity impact loading was investigated using finite element analysis and experimental tests. The cylindrical specimen subjected to the impact load was divided into different regions separated by nodes using finite element method in ABAQUS environment with boundary conditions specified. The material properties were assumed to be lower in the region where the probability of strain localization is high based on prior experimental results in order to initialize the formation of the adiabatic shear bands. The finite element model was used to determine the maximum flow stress, the strain hardening, the thermal softening, and the time to reach the critical strain for the formation of adiabatic shear bands. Experimental results show that deformed bands were formed at low strain rates and there was a minimum strain rate required for the formation of the transformed band in the alloy and the cracks were initiated and propagated along the transformed bands leading to fragmentation under the impact loading. The susceptibility of the adiabatic shear bands to cracking was markedly influenced by the strain-rates and the initial material microstructure. The simulation results obtained were compared with the experimental results obtained from the AISI 4340 steel under high strain-rate loading in compression using split impact Hopkinson bars. A good agreement between the experimental and simulation results was obtained.

深冷处理对PCBN刀具切削性能的影响研究

针对PCBN刀具在硬切削加工中磨损严重、切削寿命短的问题,开展了PCBN刀具深冷处理试验和经深冷处理的PCBN刀具高速硬车削AISI 4340钢的试验,研究深冷处理对PCBN刀具表面完整性(包括微观形貌、粗糙度、显微硬度和残余应力)及切削性能的影响规律,探究提高PCBN刀具耐磨性和刀具寿命的方法。研究结果表明,经过深冷处理之后,刀具表面缩松缩孔等缺陷明显减少甚至消失,表面形貌得到改善,但刀具表面粗糙度增大;刀具表面显微硬度、残余应力均得到提高;表面形貌和表面残余应力对刀具寿命影响较大,其次是表面显微硬度,表面粗糙度对刀具寿命影响较小。深冷处理后,刀具黏结、氧化磨损减少,刀具寿命较未处理刀具提高24.78%,可见,深冷处理可有效改善PCBN刀具表面完整性,从而提高刀具寿命。