Influence of annealing and spheroidizing treatment on microstructure and mechanical properties of AZ91 magnesium alloy

The low-strength and high-brittleness of AZ91 cast magnesium alloy mainly result from the coarse divorced eutectic phase.To solve these problems,the annealing treatment of AZ91 cast magnesium alloy was carried out at 415 ℃ and held for 24 h in this study and the alloy was then slowly cooled to room temperature in furnace.The microstructures of the alloy were observed using a metallographic microscope,a transmission electron microscopy and an emission scanning electron microscopy,respectively.The phase analysis was performed using the X-ray diffraction,and the tensile test of the specimen at ambient temperature was performed on a material test machine.The results indicate that the coarse divorced eutectic phase dissolves into the Mg matrix during the isothermal process,and the lamellar β-Mg17Al12 phase precipitates from the magnesium solid solution with a type of pearlite precipitation during furnace cooling.Consequently,the spheroidizing treatment was carried out at 320 ℃ for 20 h following the annealing process and the lamellar β-Mg17Al12 phase was spheroidized.Compared with the as-cast alloy,the strength and ductility of the AZ91 magnesium alloy are increased obviously after annealing treatment;the yield strength and tensile strength are increased to 137.8 MPa and 240.4 MPa from 102.9 MPa and 199.3 MPa,respectively;and the elongation is improved to 6.12% from 4.35%.After being spheroidized,the strength and hardness decrease a little,but the ductility is elevated to 7.23%.The nucleation,growth and spheroidizing mechanism of the lamellar β-Mg17Al12 phase were also discussed.

Microstructure Evolution and Its Effects on the Mechanical Behavior of Cold Drawn Pearlite Steel Wires for Bridge Cables

The microstructure evolution and its effects on the mechanical performance of 2000 MPa bridge cable steel wires were investigated by transmission electron microscope(TEM),electron backscatter diffraction(EBSD),X-ray diffractometer(XRD)and mechanical tests.Experimental results reveal that,with the increasing strain from 0 to 1.42,a fiber structure and a<110>fiber texture aligned with the wire axis are gradually developed accompanied by cementite decomposition and the formation of sub-grains;the tensile strength increases linearly from 1510 to 2025 MPa,and the reduction of the area is stable with a slight decline from 44%to 36%.After annealing at 450℃for different times,pronounced changes in the microstructure occur.Cementite lamella fragment into coarser globules corresponding to a remarkable spheroidization process,while ferrite domains recover and recrystallize,and this process is associated to modifications in the mechanical properties.Furthermore,based on the observations on dislocation lines crossing through cementite lamellae,a possible mechanism of cementite decomposition is discussed.

Investigation into microstructure of spray formed V4 cold working mould steel and its roiling and annealing

The material selected for this work was the spray formed Vanadis4 high alloy cold working mould steel （abbreviated to V4 steel）. Its microstructure, hot rolling process, and annealing treatment have been investigated. Observed from the optical and electron microscopes, the as-sprayed V4 steel had the finer microstructure of uniform and equiaxial grains ,while after hot rolling for densification and spheroidized annealing, the V4 steel obtained an excellent spheroidized structure that is favorable to subsequent quenching and tempering treatment. The spheroidized structure and level of annealed hardness of the V4 steel are almost the same as expensive imported powder metallurgy the V4 steel. It is difficult to produce V4 steel with the conventional ingot metallurgical technique, so the multi-step and high-cost powder metallurgy method is generally used at present. Compared to the powder metallurgy technique, using the spray forming technique to produce the V4 steel has obvious advantages and potential market competitiveness in reducing production costs, simplifying working process, and shortening the production cycle.

Carbide Evolution in High Molybdenum Nb-microalloyed H13 Steel during Annealing Process

Based on the Thermo-Calc thermodynamic software, the type of equilibrium precipitated carbides and their contents in high Mo Nb-microalloyed H13 steel （NMH13 steel） were calculated. The composition, morphology, and distribution of carbides after spheroidal annealing of two forged experimental steels were comparatively examined by means of optical microscopy （OM）, scanning electron microscopy （SEM）, electron dispersive spectroscopy （EDS） and transmission electron microscopy （TEM）. VC, M23 C6 and M6C are identified in H13 steel after spheroidizing annealing, while （V,Nb）C, M23C6 , M2C and M6C are observed in NMH13 steel. Moreover, it is found that the ad- dition of Nb significantly enhances the stability of MC phase and the high Mo content accelerates the precipitation of small rod-shape M2C phase in NMH13 steel. The amount of the fine carbides in NMH13 steel obviously increased with M2 C and M6 C precipitated from the ferrite phase, which is in accordance with the results of thermodynamic cal- culations.

Effect of Cyclic Annealing on Microstructure and Mechanical Properties of Medium Carbon Steel

The microstructure and mechanical properties of medium carbon steel after cyclic heat treatment were in- vestigated. The effects of cyclic numbers and long time annealing on the microstructure and mechanical properties of the experimental steel were compared. A short duration （5 min） holding at 1023 K （above A1 temperature） and a short-duration （3 min） holding at 893 K are adopted in each cyclic heat treatment. The spheroidization is accelerated during cyclic heat treatment, and the spheroidizing ratio grows with cyclic numbers. After 12-cycle heat treatments, there are few incompletely spheroidized regions in the specimens, and cementite lamellae mostly change into cement- ite particles. The morphological character of cementite for 12 cycles is similar to that undergoing annealing for 10 h at 973 K. The strength of the experimental steel after 5-cycle heat treatment is the lowest in the following cyclic heat treatment, but it is still higher than that of specimens with subcritical annealing over a long period （10 h）. After 12- cycle heat treatment, the strength of the experimental steel is close to that of the normalized steel, and the plasticity is the best in all heat treated specimens.

Parameters Optimization of Low Carbon Low Alloy Steel Annealing Process

A suitable match of annealing process parameters is critical for obtaining the fine microstructure of material. Low carbon low alloy steel （20CrMnTi） was heated for various durations near Ac temperature to obtain fine pearlite and ferrite grains. Annealing temperature and time were used as independent variables, and material property data were acquired by orthogonal experiment design under intercritical process followed by subcritical annealing process （IPSAP）. The weights of plasticity （hardness, yield strength, section shrinkage and elongation） of annealed material were calculated by analytic hierarchy process, and then the process parameters were optimized by the grey theory system. The results observed by SEM images show that microstructure of optimization annealing material are consisted of smaller lamellar pearlites （ferrite-cementite） and refining ferrites which distribute uniformly. Morphologies on tension fracture surface of optimized annealing material indicate that the numbers of dimple fracture show more finer toughness obviously comparing with other annealing materials. Moreover, the yield strength value of optimization annealing material decreases apparently by tensile test. Thus, the new optimized strategy is accurate and feasible.

Effect of Subcritical Annealing Temperature on Microstructure and Mechanical Properties of SCM435 Steel

The effect of subcritical annealing temperature on microstructure and mechanical properties of SCM435 steel was investigated through changing the heating and soaking temperature as 660 °C, 680 °C, 700 °C, 720 °C and 745 °C. The microstructure and mechanical properties of intercritically annealed specimens were analyzed. With increasing the subcritical annealing temperature from 660 °C to 720 °C, the spheroidization ratio gradually increased, and the mechanical properties, formability and Vickers hardness were improved. According to the comprehensive comparison of mechanical properties and formability, the subcritical process at soaking temperature of 680-720 °C could achieve similar annealing effect as that of intercritical process. Therefore, the subcritical annealing temperature could be set as 700 °C in practice, with the Ac1 temperature fluctuation within ±20 °C, and the applicability and stability of subcritical annealing were guaranteed in industrial application. The plant results of the cold heading showed that the subcritical annealing could replace original intercritical annealing successfully with significantly saving time and energy.

Cementites decomposition of a pearlitic ductile cast iron during graphitization annealing heat treatment

Cementites decomposition of a pearlitic ductile cast iron during graphitization annealing heat treatment was investigated.Fractographies and microstructures of heat treated samples were observed using a scanning electron microscope and mechanical properties were measured by a universal tensile test machine.The results indicated that during isothermal annealing at 750°C,the tensile strength of pearlitic ductile cast iron was increased to a peak value at 0.5h,and decreased gradually thereafter but the elongation was enhanced with the increase of annealing time.Moreover,the diffusion coefficient of carbon atoms could be approximately calculated as 0.56μm2/s that could be regarded as the shortrange diffusion.As the holding time was short（0.5h）,diffusion of carbon atoms was incomplete and mainly occurred around the graphites where the morphology of cementites changed from fragmentized shape to granular shape.In addition,the ductile cast iron with tensile strength of 740MPa and elongation of 7% could be achieved after graphitization annealing heat treatment for 0.5h.Two principal factors should be taken into account.First,the decomposition of a small amount of cementites was beneficial for increasing the ductility up to elongation of 7%.Second,the diffusion of carbon atoms from cementites to graphites could improve the binding force between graphites and matrix,enhancing the tensile strength to 740 MPa.

球化退火冷却速率对SR19热作模具钢组织和硬度的影响

利用DEFORM软件模拟得到大尺寸(200 mm×800 mm×2 000 mm)SR19热作模具钢在退火降温过程中不同深度位置的温度场,根据降温曲线设计该钢小尺寸试样的球化退火工艺,如下:将试样加热至860℃保温2 h,分别以不同速率(15,30,45,60℃·h^(-1))缓冷至740℃保温4 h,再炉冷至500℃后空冷至室温。分析了冷却速率对试样组织和硬度的影响。结果表明:随着冷却速率增大,试样中的先共析铁素体含量和碳化物分布均匀程度均先减小后增大,球化率先保持平稳后快速下降,退火硬度先减小后增大;当冷却速率为45℃·h-1时得到的显微组织最佳,碳化物分布均匀,球化率较高,先共析铁素体含量较少且出现了硬度的回落。先共析铁素体产生的原因主要是退火过程中冷却速率的控制不当导致奥氏体优先转变为铁素体并保留在最终组织中。

冷轧连退带钢力学性能不合格原因

采用金相检验、力学性能测试等方法,对某冷轧连退带钢屈服强度、抗拉强度、断后伸长率等力学性能指标不合格的原因进行分析。结果表明:当退火速率较慢时,明火段板带温度升高,当板带温度比珠光体开始奥氏体化温度高30℃时,组织中珠光体趋于球化转变,材料的强度明显降低;当退火速率较快时,明火段板带温度下降,当板带温度比珠光体开始奥氏体化温度低10℃时,晶粒再结晶或者晶粒长大不完全,造成材料强度偏高、断后伸长率偏小。

退火对双金属带锯条焊接接头组织演变及元素分布规律的影响

双金属带锯条由M42高速钢齿材和B2000高强度钢背材焊接而成,焊接后易产生焊接热应力,影响其焊缝性能。研究了双金属带锯条焊接接头经过820℃(退火炉中)退火再随炉冷却后的组织演变和元素分布规律。结果表明,退火后的焊缝组织由柱状晶逐渐演变为碳化物球化,Mo、Cr、V、W等合金元素与C形成碳化物并逐渐球化,导致合金元素在接头处团聚。此外,退火过程中元素扩散系数增大,使得双金属带锯条的母材中的元素向焊缝中扩散,形成了M6C型、M23C6型和MC型碳化物,降低了焊缝硬度。该研究结果为双金属带锯条焊接接头的退火工艺优化提供了理论依据,有助于提高双金属带锯条的质量和使用寿命。

含硼冷镦钢10B33冷镦开裂原因分析

含硼冷镦钢10B33冷镦锻打带法兰面六角螺栓时出现法兰面开裂。对冷镦开裂不同形貌的螺栓、未开裂螺栓、拉拔精丝及母材盘条分别进行检验。结果表明:法兰面圆周上存在多处碎裂,较大裂口呈现斜向45°开裂形貌的冷镦螺栓是由于球化退火工艺不良,法兰界面存在较多未完全球化的珠光体团,球状珠光体弥散不完全,材料塑性不足引起的切应力开裂;法兰圆周单一竖直开裂形貌的冷镦螺栓,由于铸坯近角部存在卷渣缺陷,轧制成盘条后缺陷无法消除,带有缺陷盘条拉拔制成的精丝冷镦时卷渣缺陷扩展造成开裂。

20CrMnTi钢球化退火工艺优化

对比分析两种退火工艺下材料的组织和性能,获得最佳球化退火工艺参数。将试样随炉加热至715℃后以20℃/h速度升温至760℃保温7 h。随炉冷却至720℃保温5 h,再以20℃/h速度降至500℃,然后自然冷却,球化级别达到4级,碳化物75%为球状或点状,布氏硬度136 HBW,满足客户使用要求。还需优化材料的塑性、硬度及强度指标,优化保温时间和保温温度,以降低能耗,改善冷挤压效果。

罩式炉GCr15钢丝球化退火工艺研究

对比分析了GCr15轴承钢普通球化退火工艺和改进球化退火工艺处理后的金相组织和硬度。结果表明,GCr15轴承钢采用改进球化退火工艺处理后比普通球化退火处理后硬度值低6 HBW,金相组织级别波动小,经0.2~0.3 mm拉拔减径后,成品材料硬度满足要求。改进球化退火工艺采用氢气作为保护气氛,材料无附加脱碳;采用快速加热方式,工艺周期缩短2 h,材料金相组织和硬度均匀性较改进前明显提高;生产效率提高,经济效益显著。

不同冶炼方式下的H13的锻造生产及组织性能对比

随着新能源汽车的快速发展,急需对高热疲劳性、高使用寿命的铝压铸模具钢材进行研发应用。而H13属于热作模具钢,普遍应用于铝挤压、铝压铸、热模锻等行业,市场需求量较大。主要根据原材料冶炼方式的不同,对H13锻圆的锻造生产以及组织性能进行对比分析。

退火工艺对低碳齿轮钢球化退火组织影响

通过对低碳20CrMnTiH齿轮钢在不同保温温度及保温时间进行直接淬火和球化退火试验,分析了退火温度及时间对齿轮钢球化退火组织及硬度的影响。

亚温球化退火工艺对热轧态65Mn钢组织及性能影响

主要研究了亚温球化退火工艺对热轧态65Mn钢的组织及性能的影响,通过模拟不同退火温度、保温时间、冷却速率等条件,得到热轧态65Mn钢组织及性能的变化规律,为优化65Mn钢罩式退火工艺提供参考。结果表明,当温度低于相变点温度时,退火温度越高,热轧态65Mn钢球化效果越好;随着保温时间的延长,片状珠光体逐渐趋于球化,720℃保温12 h后,继续延长保温时间对球化效果影响不大;另外,在相同的加热温度和保温时间下,冷却速率对热轧态65Mn退火后的组织及性能无明显影响。

SWRCH22A自攻螺钉扭矩异常分析

通过对扭矩不合格自攻螺钉低倍组织和断口形貌的检验分析,总结了SWRCH22A自攻螺钉加工过程中导致扭矩不合格的原因。认为热轧盘条残余缩孔、奥氏体混晶组织以及球化退火、渗碳淬火热处理工艺不当降低了自攻螺钉扭矩,影响其使用性能。针对性提出了热轧盘条生产过程中连铸头尾坯及冶炼深脱氧关键工艺的控制要求和自攻螺钉加工过程中无检验手段调整热处理工艺的判断方法等措施。采取措施后,热轧盘条的致密度和均匀性满足加工要求,自攻螺钉获得硬度梯度良好的浅层渗碳淬火组织,扭矩合格。

不同球化退火工艺对T12A组织与力学性能影响的研究

讨论了T12A碳素工具钢球化处理原理与碳化物球化的形成机理,通过典型球化处理试验,优化了球化处理工艺参数。结果表明,采用770℃的复合球化退火工艺后,T12A碳素工具钢的碳化物颗粒分布弥散,晶粒尺度为1.5~1.8μm,硬度为92~94 HRB。获得了具有实用价值的球化处理工艺,对于高效益生产中T12A碳素工具钢的球化处理具有重要参考价值。

低温过热器管开裂原因

某化工厂燃油锅炉低温过热器管发生开裂,采用宏观观察、化学成分分析、金相检验、扫描电镜分析、力学性能测试等方法分析了开裂原因。结果表明:低温过热器管开裂的原因是管壁材料珠光体球化严重,使管壁迎火面的抗拉强度和屈服强度下降,管壁的耐热性能下降,从而导致低温过热器管发生开裂。