Morphology of bainite and widmanstätten ferrite

Morphology of bainite and Widmanstätten ferrite in various steels has been investigated by means of microstructural and surface relief observations. It was shown that upper and lower bainite should be classified by ferrite morphology,i.e., lathlike or platelike, and that the morphology of cementite precipitation cannot be the index for the classification. Widmanstätten ferrite formed in the upper C-nose where ferrite grain-boundary allotriomorphs nucleate exhibits quite similar appearance with bainitic ferrite that forms in the lower C-nose of bainitic reaction. The only difference between them exists in the fact that Widmanstätten ferrite laths grow in the temperature range where primary ferrite forms and often terminate at a grain boundary ferrite but that bainitic ferrite has its own C-curve at temperatures belowB _s and nucleates directly at an austenite grain boundary. The mechanisms for their formations are discussed.     

两相区位错增殖对低碳贝氏体/铁素体复相钢组织和性能的影响

利用扫描电子显微镜(SEM)、透射电子显微镜(TEM)、电子探针(EPMA)、X射线衍射仪(XRD)、室温拉伸等手段,通过两相区保温-淬火(IQ)、两相区形变后保温-淬火(DIQ)、两相区保温-淬火-配分-贝氏体区等温(IQ&PB)及两相区形变后保温-淬火-配分-贝氏体区等温(DIQ&PB)热处理工艺,研究高温形变对室温组织、性能、残余奥氏体稳定性的综合影响作用.结果表明,经15%的压缩形变后铁素体中位错密度由0. 290×1014增加至1. 286×1014m-2,马氏体(原奥氏体)中C、Cu元素富集浓度提高,高温形变产生位错增殖对元素配分有明显促进作用. DIQ&PB工艺下,形变后贝氏体板条尺寸变短且宽度增加0. 1μm左右,贝氏体转变量较未变形时增加14%,多边形铁素体尺寸明显减小.力学性能方面,两相区形变热处理后抗拉强度增加132. 85 MPa,断后伸长率增加7%,强塑积可达25435 MPa·%.形变后残余奥氏体体积分数由7. 8%提高到8. 99%,残余奥氏体中碳质量分数由1. 05%提高到1. 31%.     

低碳贝氏体钢低温CTOD试验断口分离分析

通过对低碳贝氏体钢在-20℃CTOD试验中出现的分离断口进行微观分析,研究了断口分离和失稳断裂的原因.结果表明,断口分离面均为脆性断裂,至主断面为韧性扩展,分离裂纹产生的主要因素不是夹杂物,而是材料内部的带状组织和成带状的硬相组织受三维应力作用的结果.拉伸材料的分离裂纹是在达到一定抗拉强度之后开始萌生和扩展,不影响材料的使用性能.在断裂韧度试验中,一方面分离裂纹能降低材料裂纹尖端的三维应力约束,提高材料的韧性;另一方面较大程度的分离裂纹,减小了裂纹形核功和扩展功,诱发主裂纹的失稳扩展.     

低碳贝氏体工程机械用Q460级厚钢板的研制

在工业试制条件下,通过合理的化学成分设计和TMCP工艺设计,采用细晶强化、析出强化和贝氏体组织强化等手段,研制了厚度为65~80 mm的低碳贝氏体工程机械用Q460级厚钢板。试制钢板具有良好组织及力学性能,同时降低了生产成本。     

Structure and mechanical properties of hot-deformed low-carbon martensitic steel

The structural changes in low-carbon martensitic 15Kh2G2NMFBA steel induced by its hot forging in the temperature range 1150–850°C have been studied. The calculated cracking resistance parameter I_c is in agreement with its experimental value. A relation is found between the lath sizes in the martensite structure and the change in the impact toughness characteristics. A combined regime of hot deformation and hot treatment of the low-carbon martensitic steel is proposed to form submicrometer-sized structural elements and high strength and impact toughness characteristics.     

Retained austenite and mechanical properties in bainite transformed low alloy steels

Uniform ductility and formability of low alloy steels can be improved by the transformation plasticity effect of metastable retained austenite. In this work, intercritical annealing followed by bainite transformation resulted in the retention of austenite with sufficient stability for transformation plasticity interactions. The effect of retained austenite on mechanical properties was studied in two low-alloy steels. Bainite transformation was carried out in the range of 400 to 500°C. The strength properties (yield strength and ultimate tensile strength) were more sensitive to bainite isothermal transformation temperature than holding time. Maximum strength properties were obtained for the lower transformation temperatures. On the other hand, high uniform and total elongation values were obtained at lower transformation temperatures but were sensitive to bainite isothermal transformation time. Variations in uniform elongation with holding time were linked to variations in retained austenite stability. Maximum values of uniform elongation occurred at the same holding times as the maximum amount of retained austenite. The same was true for total elongation and ultimate tensile strength. The above results indicate a strong correlation between retained austenite stability and uniform ductility and suggest that further optimisation regarding chemical composition and processing with respect to austenite stabilisation may lead to a new class of triple-phase high-strength high-formability low-alloy steels.     

Effect of bainite on the mechanical properties of dual-phase steels

The bainite-martensite-ferrite steels (tri-phase steels) were made in the laboratory by intercritical annealing, bainite transformation and oil quenching in sequence. With bainite inclusions, ductility was improved substantially without significant reduction of tensile strength. The ductility increase was found to be due to large deformation after necking and increased work-hardening.     

Nonuniform structure and properties in hot-rolled low-carbon steel coils

The austenite phase transitions in the industrial production of hot-rolled steel coils are studied magnetometrically. Nonuniformity of the structure and mechanical properties of low-carbon steel strip over its length and width may be attributed to incomplete phase transitions in the strip on the output roller conveyer and nonuniformity of the subsequent coil cooling in regular production conditions. Inheritance of the structural inhomogeneity formed after hot rolling may be responsible for the thickness variation observed after cold rolling of the strip. Recommendations are made for reducing or eliminating the structural inhomogeneity so as to improve product quality.     

Characteristic properties of continuous annealed low-carbon low-aluminium steels

Continuous annealed low-carbon low-aluminium steel grades with good aging resistance and bake-hardenability were investigated. Taking into account the amounts of solute carbon and nitrogen in supersaturated ferrite it was tried to optimize the steel composition. Aging sensitivity depends mainly on solute nitrogen though the contribution to aging of solute carbon cannot be neglected. A reduced nitrogen aging requires a low nitrogen content (20 ppm) as well as Al/N ratios not lower than 5 and carbon contents higher than 0,025%. Both, low amounts of solute carbon and decrease of ferrite grain sizes of steels coiled at 650°C increase the effect of bake hardening. In comparison with aged steels the yield strength of overaged steels is conservative after baking. Bake-hardenable steels with good aging resistance are obtained in the carbon range 0,025 to 0,030%. Optimization according to the r?-value is not achieved in this range.     

On the structure and properties of high-nitrogen low-carbon non-austenitic steels

Phase transformations, when cooling and heating non-austenitic high-nitrogen low-carbon steels containing chromium and other alloying elements, as well as structure and mechanical properties of these steels were analyzed. It was confirmed that these steels have high temperature chromium diffusion controlled pearlitic type transformation and martensitic type transformation. Experimental high nitrogen steels after quenching and tempering provide mechanical properties of about the same level as high strength commercial alloyed steels. Features of nitrogen as an alloying element in steels discussed allow the supposition of a possible reduction of the consumption of nickel, manganese, molybdenum or tungsten in high strength alloyed steels.     

Structure and properties of low-carbon pipe steel after pneumatic testing

The structure and properties of low-carbon pipe steel with ferrite-bainite structure subjected to pneumatic tests are investigated. The relation between the properties determined in static and dynamic loading and the results of the pneumatic tests is considered. It is found that the structure of steel not subjected to pneumatic tests includes large bainite sections and coarse martensite-austenite inclusions.     

The mechanical properties of α-phase low-carbon Fe-Mn alloys

An attempt has been made to assess the potential of low-carbon Fe-Mn alloys as a replacement for nickel steels, by measuring the susceptibility of lath martensites in the 4 to 10 pct Mn range to brittle fracture. The alloys were all brittle, but this was not caused by any inherent effect of manganese on the flow properties of iron, such as changes in the σ_i orK _y parameters. The most likely cause of brittleness was found to be a classical temper brittleness effect. J. D. BOLTON, formerly Senior Lecturer, Sheffield Polytechnic, Sheffield, England.     

On bainite formation

Driving force calculations for Fe-C, Fe-X-C, and Cu-Zn alloys show that the formation of bainite by a shear mechanism is thermodynamically impossible. There exist superledges on the broad faces of bainite in steels, revealing that the thickening of bainite probably proceeds by a ledge mechanism. In some Fe-Ni-C alloys and commercial steels, no simple relationship was found between the strength of austenite and theBs temperature; however, there is a linear relationship betweenBs and the diffusion coefficient of carbon and iron in austenite, as well as between the incubation period and the function containing D _Fe ^γ . The bainite reaction seems to be controlled by diffusional processes. In a low-carbon Ni-Cr steel, the morphology of the bainite/matrix interface boundary is different from that of the martensite, and the habit plane of the bainite (1 7 11)_α deviates 13.3 deg from that of the martensite (1 1 0)_α, indicating that the mechanism of the bainite reaction is not necessarily analogous to that of the martensitic transformation. At temperatures nearMs, as the driving force will be large enough, the growth of bainite by shear may be able to occur, and evidence is given by the morphology of bainite showing shear characteristic. X-ray diffraction study in Ag-Cd and internal friction measurements in Ag-Cd and Cu-Zn-Al alloys, 18CrNiWA steel, and its decarburized specimen reveal that the nucleation process occurs within the incubation period of bainite formation. This paper is based on a presentation made in the symposium “International Conference on Bainite” presented at the 1988 World Materials Congress in Chicago, IL, on September 26 and 27, 1988, under the auspices of the ASM INTERNATIONAL Phase Transformations Committee and the TMS Ferrous Metallurgy Committee.     

Strain tempering of bainite

In the present investigation the strain tempering of bainite has been carried out with (EN 24) a medium carbon low alloy steel. The specimens were austenitized at 850°C and isothermally transformed at 300°, 360°, and 400°C to produce bainite, strained and tempered in the range 100° to 400°C. The bainite formed at 360°C has been specifically examined by thermomagnetic analysis to confirm quantitatively the solution of ∈ carbide. Thermomagnetic analysis has shown that the carbide phase in bainite formed even at 360°C is a mixture of ∈ carbide and cementite. It has been found that with lowering of bainite transformation temperature, the strain tempering treatment results in higher strength consistent with good ductility. The present investigation favors the carbon dislocation trapping model for the mechanism of strain tempering of bainite, similar to that proposed for the strain tempering of martensite.     

Morphology and structure of bainite plates in the β’ phase of an Ag-45 at. pct Cd alloy

The morphology and structure of bainite plates which formed in the ordered bcc β’ phase of a Ag-45 at. pct Cd alloy at temperatures 160 to 300°C were studied by X-ray diffraction and transmission electron microscopy. Initially, the plates formed with a 3R stacking fault modulation of the fcc structure and contained a high density of random stacking faults. The stacking faults annealed out during a prolonged isothermal treatment, the structure gradually changing to a regular fcc. The orientation relationship between the bcc matrix and the fcc bainite was as follows: [1•11]_b 0.7 deg from [0•11]_f, [110]_b 1.1 deg from [l00]_f and [011]_b 4.3 deg from the stacking fault plane pole [111]_f. The habit plane of the bainite plates, determined by two surface trace analysis, was close to (3, 11, 12)_b. The surface relief of the plates, examined by interference microscopy, was in the form of a simple tilt indicating an invariant plane strain transformation. The features of the transformation agreed with the predictions of the Bowles-MacKenzie theory of martensite formation. It was concluded that the morphology, structure and other characteristics of the freshly formed bainite plates were consistent with their formation by a thermally activated martensitic process. formerly Research Associate, Department of Metallurgy, University of British Columbia, Vancouver, B.C., Canada.