Investigation of SUS304 Stainless Steel with Warm Hydro-mechanical Deep Drawing

Basing on warm mechanical property of SUS304 stainless steel and hydro-mechanical deep drawing process, warm hydro-mechanical deep drawing process is proposed and discussed with experiments in this paper. The experiments are performed at four different temperatures. The results show that the formability of stainless steel is improved under the condition of warm temperature. Warm hydro-mechanical deep drawing raises limiting drawing ratio of SUS304 effectively, and limiting drawing ratio 3.3 is obtained, which is beyond 2.0 with conventional deep drawing. The temperature of 90℃ is beneficial to the forming of SUS304 stainless steel, the strain-induced martensite is controlled effectively, and the thickness distribution is more uniform.

Residual stress in the cylindrical drawing cup of SUS304 stainless steel evaluated by split-ring test

The residual stresses in the wall of a SUS304 stainless steel cylindrical drawing cup were evaluated by split-ring tests, and the influences of stamping die parame- ters on the residual stress were investigated. A new theoretical model of a split-ring test was developed to evaluate the resid- ual stress in a ring, which was verified to be reasonable and reliable by numerical simulations with ABAQUS code and by nanoindentation tests. Seven groups of split-ring tests were completed, and the residual stresses were calculated according to the theoretical model. The split-ring test results showed that the circumferential residual stress in the wall of the SUS304 stainless steel cylindrical drawing cup was very large and did not change with the different die comer radius. The circumferential residual stress first increased with the increase of drawing punch-die clearance, then was almost unchanged when the clearance increased greater than blank thickness 1 mm. Thus, a smaller clearance was suggested to be chosen to reduce the residual stress in the wall of the SUS304 stainless steel drawing cup.

Investigation on the Hydro-forming of SUS304 Stainless Steel Dome

The dome hydro-forming was studied theoretically and experimentally. The theoretical analysis shows that the process of dome hydro-forming can be divided into two stages, i.e., firstly forming a hemi-sphere and secondly forming a hemi-toroidal. Theoretically, being larger than those in the outer area of a hemi-toroidal shell, the radial tensile stresses take place in the inner area where fracture is more likely to occur in the forming process. The required hydraulic pressure to form the dome should be determined by the deformation in the outer area, hitting about 12 MPa in theory. The experimental results show that the dome could be produced in one operation by hydro-forming which needs a hydraulic pressure of about 9 MPa. The experimental results prove to agree well with the theoretical analysis. In order to avoid wrinkling and fracture, the closing force acted on the die should be adjusted properly in accordance with the hvdraulic nressure.

脉冲电流对SUS304不锈钢拉伸性能的影响

通过SUS304亚稳态奥氏体不锈钢的电塑性拉伸实验,分析了焦耳热效应和纯电塑性效应对SUS304不锈钢拉伸性能的影响。结果表明,脉冲电流可以有效降低材料的流动应力。焦耳热效应使材料的延伸率随电流密度的增大而减小;纯电塑性效应使材料的延伸率增加。当消除焦耳热效应时,电流密度为2.95A·mm-2的试样延伸率达到72.4%,比室温时增加了23.3%。基于拉伸过程中的能量关系推导出电塑性效应系数的求解公式,发现电塑性效应系数与电流密度、应变以及试样尺寸等因素有关,电流密度越大,电塑性效应系数越大。

Influence of Pre-Transformed Martensite on Work-Hardening Behavior of SUS 304 Metastable Austenitic Stainless Steel

The metastable austenite was transformed to martensite by prestrain tension of SUS304 stainless steel to study the influence of transformed martensite on its subsequent work-hardening behavior under the uniaxial tensile condition. The X-ray diffractometer （XRD） was employed to detect the transformed martensite. Results showed that the volume fraction of transformed martensite increases with increasing prestrain. The pre-transformed martensite in the microstructure remarkably affects the deformation behavior of the steel, and the strength increases and the elongation decreases. The work-hardening curve of prestrained specimens observably changes with true strain. The work-hardening exponent n of stainless steel decreases with the increase of pre-transformed martensite. The achievement is a significant contribution to the process design during pressing.

Influence of Strain Rate on Tensile Characteristics of SUS304 Metastable Austenitic Stainless Steel

The microstructure characteristics and plastic deformation behavior of SUS304 metastable austenitic stainless steel sheets have been investigated during tensile process at different strain rates at room temperature. The yield stress continuously increases with strain rates due to low fraction of martensite transformed from austenite at 0.2% plastic stain. While the ultimate tensile stress （UTS） and elongation gradually decreases and then slightly increases with increase in strain rate from 0.0005 s-1 to 0.i s-1, which is attributed to the variation of the martensite fraction that is affected seriously by adiabatic heating. A higher temperature increase in the tensile specimens restricts the martensitic transformation at high strain rate. The strain rate of 0.1 s-1 is considered as a transition deformation rate from quasi-static state to plastic forming, where the transformed martensitic content is very small in a higher strain rate range. Anomalous stress peaks in the later half stage of deformation occur at a very low strain rate （i.e., 0.0005 s-1） result from X-shaped strain localization repeatedly sweeping over the specimen. With increasing strain rates, the variation of dimple number density follows similar trend as that of UTS and ductility because martensite fraction mostly influences void nucleation and growth.

In Situ TEM Observation on Martensitic Transformation during Tensile Deformation of SUS304 Metastable Austenitic Stainless Steel

Through in situ transmission electron microscopy observation on SUS304 metastable austenitic stainless steel during stretching at room temperature,it is found that e martensite plates were induced preferentially from the sites of dislocation pile-ups.With increasing deformation,some of ε thin martensite platelets disappear and reversibly transform toγ austenite without heating treatment,which is different from the previous result that ε martensite can entirely transform toα＇martensite.Then,some of deformation twins appear and grow along the vertical direction of ε martensite due to（111）_γ⊥（1010）_ε.Moreover,it is directly observed that multiple transformation mechanisms via γ→ε→γ,γ→ε,γ→α′,γ→ε→α′,γ→ deformation twins →α′ can co-exist.

基于PLC控制的T2紫铜-SUS304不锈钢激光焊接组织与性能研究

采用激光深熔焊方法对T2紫铜和SUS304不锈钢进行了焊接处理,研究了基于PLC控制的光束偏移对焊接接头显微组织和拉伸性能的影响。结果表明:不同光束偏移量的T2紫铜-SUS304不锈钢焊接接头的剖面形貌中均呈现铜侧平直、钢侧沙漏状形态;光束正偏移时焊缝较窄、负偏移时焊缝较宽;光束正偏移时的铜侧热影响区宽度明显大于光束负偏移(光束偏向钢侧)时,光束正偏移时钢侧熔合线附近的胞状枝晶较小且存在较多的颗粒状Cu2O相,光束负偏移时熔合线附近为明显垂直于焊缝生长的细长柱状晶以及靠近焊缝中心的网状胞晶,局部可见颗粒状Cu2O相;光束偏移0和+0.15mm的焊接接头断裂位置在焊缝处,光束偏移+0.30mm、-0.15mm和-0.30mm的焊接接头断裂位置都处于铜侧热影响区;光束偏移+0.3mm时可以取得较好的焊接效果,焊接接头的拉伸断口形貌观察结果与拉伸测试结果相吻合。