Analysis and prevention of microcracking phenomenon occurring during strip casting of an AISI 304 stainless steel

This study was concerned with the effects of microstructural parameters on the microcracking phenomenon occurring during strip casting of an AISI 304 stainless steel. Detailed microstructural analyses of the microcracked regions showed that microcracks were formed mainly along tortoise-shell-shaped depressions and that their number and size were considerably reduced when strip casting was done right after a shot-blasting or pickling treatment of the casting roll surface. This microcracking phenomenon was closely related to the formation of a black oxide layer, which was mainly composed of manganese-rich oxides, on the roll surface. The black oxide layer acted as a barrier of thermal transfer between the rolls and melt, led to an increased gas gap and inhomogeneous solidification of cast strips, and, thus, played a role in forming both tortoise-shell—shaped depressions and microcracks on the strip surface. The installation of brush rolls behind the casting rolls was suggested as a method to prevent microcracks, because the brush rolls could continuously scrape off the black oxide layer affixed on the roll surface during strip casting.     

Analysis and prevention of cracking during strip casting of AISI 304 stainless steel

In this study, a microstructural investigation was conducted on the cracking phenomenon occurring during strip casting of an AISI 304 stainless steel. Detailed microstructural analyses of the cracked regions showed that most of the cracks were deep, sharp, and parallel to the casting direction. They initiated at the tip of dendrites and propagated along the segregated liquid films between primary dendrites, indicating that they were typical solidification cracks. This cracking phenomenon was closely related to the inhomogeneous solidification of cast strips, represented by depressions, i.e., uneven and somewhat concave areas on the strip surface. The depressions, which were unavoidable in flat rolls due to the presence of a gas gap between the roll and the cast strip, were finely and evenly distributed over the cast strip surface by intentionally providing homogeneous roughness on the roll surface; then, the number and size of cracks were considerably reduced. In addition, the nitrogen gas atmosphere, which retained high solubility in the melt during cooling and good wettability with the roll surface, was successfully used to prevent cracking, because the thickness of the gas gap was minimized.     

冷墩钢冷墩开裂原因分析

对ML42CrMo、SWRCH22A、SWRCH6A等冷墩钢的冷墩开裂试样进行金相检验和电子探针分析。结果表明，材料的表面缺陷、内部聚集分布的夹杂物、较严重的成分偏析或组织不正常等都可以成为冷墩开裂的原因。     

铆螺钢冷镦裂纹形成与消除的研究

通过理化检测,对铆螺钢在冷镦时出现开裂现象的原因进行了分析.结果表明: 铆螺钢在冶炼和浇注过程中生成的复合夹杂物,造成其在冷镦过程中开裂的发生.通过调整连铸前的吹氩工艺和中间包的烘烤工艺,有效地避免了铆螺钢冷镦开裂现象的发生.     

中碳冷镦钢冷镦爆裂原因分析

中碳冷镦钢是中、高强度紧固件的主要原材料,连铸过程中铸坯表面增碳,将会造成线材成分偏析、组织异常,镦打过程中严重开裂。     

Analysis and prevention of vertical cracking phenomena during deep drawing of hot- rolled sg295 steel strips

Microscopic examination and microstructural analysis of vertical cracking phenomena in deep-drawn cups of hot-rolled SG295 steel strips were carried out in this study. Microvoids initiated preferentially at grain-boundary carbides were observed to form intergranular cracks. These grain-boundary car-bides were identified as (Fe,Mn)₃C carbides. The morphology of carbides was varied with processing variables,e.g., killing method and coiling temperature. In the Al-killed steels, the carbide shape tends to change from film type to bulk type, which may be beneficial to elongation and consequently formability. In addition, as the coiling temperature increased, the amount of banded pearlite structures and bulk-type carbides increased. These findings suggest that the deep drawability can be improved by modifying the carbide morphology in the microstructures.     

冷镦钢开裂的原因分析及改进

为了研究冷镦钢开裂的原因和过程,取样分析了某钢厂生产中存在开裂的试样,利用带有能谱仪的扫描电镜(SEM EDS)观察开裂区域宏观形貌和微观组织并表征夹杂物的形貌和成分。结果发现,部分魏氏体混杂在基体中,降低了冷镦钢的塑性和韧性;开裂区域发生穿晶断裂和氧化脱碳现象,其完全脱碳层的平均厚度为28 μm,说明表面缺陷在轧制前就已经存在,导致冷镦钢轧制时开裂;残留在钢中的铝镁钙复合夹杂物也直接导致了表面缺陷和开裂。同时,提出改善微观组织、减少夹杂物的改进措施。     

Twinning-induced sluggish evolution of texture during recrystallization in AISI 316L stainless steel after cold rolling

This paper deals with the evolution of texture in AISI 316L austenitic stainless steel during annealing after 95 pct cold rolling. After 95 pct cold rolling, the texture is mainly of the brass type {110}〈112〉, along with a scatter toward the S orientation {123}〈634〉 and Goss orientation {011}〈100〉. Weak evidence of Cu component is observed at this high deformation level. During annealing, recovery is observed before any detectable recrystallization. Recrystallization proceeds through nucleation of subgrain by twinning within the deformed matrix and, later, preferential growth of those to consume the deformed matrix. After recrystallization, the overall texture intensity was weak; however, there are some discernible texture components. There was no existence of the brass component at this stage. Major components are centered on Goss orientation and Cu component {112}〈111〉 as well as the BR component {236}〈385〉. Also, a few orientations come up after recrystallization (i.e., {142}〈2−11〉 and {012}〈221〉). With increase in annealing temperature, the textural evolution shows emergence of weak texture with another new component, {197}〈211〉. The evolution of texture was correlated with the deformation texture through twin chain reaction.     

火花源原子发射光谱法分析冷镦钢中痕量钙和硼

用铣样机进行试样表面处理,试验后选取2 mm 的切削深度,采用火花源原子发射光谱法进行冷镦钢中痕量钙和硼的测定。优化仪器分析条件,采用进口标准样品建立钙和硼的校准曲线。钙和硼的检出限均为0.2 μg/g,适用于冷镦钢中质量分数分别为0.000 1%~0.015%的钙和0.000 1%~0.012%硼的测定。对编号为B.S. CA1A 的钙硼低合金钢标准样品进行精密度考察,当钙、硼质量分数为0.002 1%和0.001 0%时,其相对标准偏差分别为3.5%和3.9%。对钙硼低合金钢标准样品和冷镦钢实际样品进行分析,测定值与认定值及湿法的测定值基本一致。     

Influence of die geometry on undershooting phenomenon and bending phenomenon in cold extrusion of steel tube

Abstract

Numerical and experimental investigations were carried out on the excessive decrease in outside diameter occurring on a tube cold extruded through two conventional types of die, a taper die and an R die. Wide ranges of the parameters of die profile were adopted in order to precisely examine the influence of die geometry on the undershooting phenomenon, and appropriate die geometries for the suppression of the undershooting phenomenon were shown. The R die is more practical in the manufacturing operation compared with the taper die. Bending was also investigated, and it was shown that the bending intensity becomes lower as the undershooting intensity decreases. However, it is not necessary to suppress bending sufficiently, even if the undershooting is suppressed to zero in the case of adopting the ordinary types of die – there seemed to be another governing factor for the occurrence of bending and this is a remaining problem.     

20钢Φ180连铸坯表面纵裂纹的形成与防止

从一冷、保护渣方面分析了２０钢φ１８０连铸坯表面纵裂纹的形成原因,探讨了防止裂 措施,使２０钢φ１８０铸坯表面裂纹出现率由原来的３９．８％降为０．３０％。     

Phase evolution of AISI 321 stainless steel during directional solidification

Abstract

The phase evolution of AISI 321 stainless steel was studied by directional solidification and quenching techniques. Two interfaces, solid/liquid and the peritectic reaction interface, were found to exist in the directional solidification structure. With increasing growth velocity the solid/liquid interface changed in the sequence of planar, cellular, dendritic and the primary phase changed from austenite to ferrite. The phase and morphology selection was verified by the interface response functions (IRFs) and the maximum growth temperature criterion. The ferritic island banding structure was observed, not only in the austenite cellular primary growth condition (3 μm s^?1), but also in the dendritic ferrite primary growth one at relatively low growth velocity (5 μm s^?1). It is deemed that the former resulted from the nucleation of ferrite in the continuous matrix of austenite phase, yet the latter is the residual primary ferrite attributed to the growth of austenite. Both of them do not come from the nucleation near the solid/liquid interface.     

Effect of threshold stress intensity on fracture mode transitions for hydrogen-assisted cracking in AISI 4340 steel

Fracture mode transition in hydrogen-assisted cracking (HAC) of AISI 4340 steel has been studied from an equilibrium aspect at room temperature with 8.6-mm-thick double cantilever beam (DCB) specimens. The threshold stress intensity,K _th , necessary for the occurrence of HAC and the corresponding fracture surface morphology have been determined as a function of hydrogen pressure and yield strength. The K _th increases with decrease in hydrogen pressure at a given yield strength and also with decrease in yield strength at a given hydrogen pressure. AsK _th increases, the corresponding HAC fracture mode changes from the intergranular (IG) and quasi-cleavage (QC) modes to the microvoid coalescence (MVC) mode. The experimental results indicate that the critical hydrogen concentration for crack extension in the IG mode is higher than that for crack extension in the MVC mode. The fracture mode transition with varying hydrogen pressure and yield strength is discussed by simultaneously considering the micromechanisms for HAC and the hydrogen pressure and yield strength dependencies ofK _th .     

The effects of cold working on sensitization and intergranular corrosion behavior of AISI 304 stainless steel

The effects of prior cold rolling of up to an 80 pct reduction in thickness on the sensitization-desensitization behavior of Type AISI 304 stainless steel and its susceptibility to intergranular corrosion have been studied by electrochemical potentiokinetic reactivation (EPR) and Strauss-test methods. The results indicate that the prior deformation accelerated the sensitization as compared to the undeformed stainless steel. The deformed Type 304 stainless steel experienced desensitization at higher temperatures and times, and it was found to be enhanced by increased cold deformation. This could be attributed to the increased long-range chromium diffusion, possibly brought on by increasing pipe diffusion and vacancies. The role of the deformation-induced martensite (DIM) and texture, introduced by uniaxial cold rolling, on the sensitization-desensitization kinetics has also been discussed. This study could not reveal any systematic relationship between texture and the degree of sensitization (DOS) obtained. The effect of DIM on DOS seems to be pronounced at 500 °C when the steel retained significant amounts of DIM; however, the retained DIM is insignificant at higher sensitization times and temperatures.     

Correlation of microstructure and surface roughness of disc drums fabricated by hot forging of an AISI 430F stainless steel

The surface roughness of disc drums fabricated by hot forging of an AISI 430F stainless steel was investigated. Emphasis was placed on the role of microstructural changes, which depend on postforging annealing conditions. In the forged drum specimens, MnS inclusions were heavily elongated, and a large amount of martensite was found. Three different annealing conditions were used on the forged drum specimens to completely or partially decompose martensite. The surface roughness tests were conducted on these specimens, and then the test data were compared via hardness and microstructures. The top plane of disc drums, where MnS inclusions were elongated perpendicular to the machined surface, showed a good surface roughness because of the free-machining effect of MnS. However, the side plane, where MnS inclusions were aligned parallel to the machined surface, showed an increased surface roughness due to reduction of the MnS effect. These findings suggested that a proper amount of martensite formed during forging and annealing would be beneficial for cost efficiency, as well as machinability of the 430F stainless steel products.     

Behavior of sulfide inclusions during thermomechanical processing of AISI 4340 steel

The morphological and compositional modifications of sulfides in AISI 4340 low alloy steel, in which the sulfur level was raised to about 0.1 pct, were studied during hotrolling at 1223 K followed by homogenization at 1583 K for various lengths of time. The relative plasticity of sulfide inclusions with respect to the steel matrix increased with cooling rate during solidification, hence with iron content. The number of sulfides first increased with homogenization time, reached a maximum and subsequently decreased. Inclusion size exhibited the opposite variation. The sulfide matrix interface area per unit volume of sulfide decreased continuously with homogenization time. These variations were in agreement with observed morphological modifications of sulfides during homogenization. During early stages of homogenization the flattened and elongated sulfide plates in the as-rolled material coarsened and became cylindrical. The cylindrical sulfides, broken into segments which spheroidized and coarsened with time, assumed finally a faceted morphology. During homogenization iron was rejected from the sulfide phase into the surrounding matrix, whereas manganese was accepted, causing the formation of manganese depleted zone around the inclusions. This paper is based on a Ph.D. Thesis submitted by Y. V. Murty to the Department of Metallurgy, University of Connecticut.