【作者】 李美艳；

【导师】 王勇；

【作者基本信息】 中国石油大学 ， 材料学， 2010， 博士

【摘要】 高铬铸钢由于碳及合金元素的含量较高,具有较高的强度及耐磨性,广泛用作热轧辊。然而其组织中含有大量的一次碳化物,呈网状沿晶界分布,致使轧制过程中产生的裂纹极易沿晶界扩展。另外,轧制过程中高铬铸钢中碳化物与基体的氧化协同性较差,高温条件下发生氧化磨损,恶化了热轧辊的高温耐磨性能,导致轧辊过早失效。本文在高铬铸钢表面进行激光熔凝处理,对熔凝层的微观组织特征进行分析,探讨搭接参数对组织和性能的影响,深入研究激光熔凝处理后的高温氧化行为、高温磨损性能及二者之间的内在联系。高铬铸钢组织由回火马氏体和大量网状的M7C3碳化物组成。当采用P=2700W、v=300mm/min、搭接率为33.3%的工艺参数进行激光熔凝处理,可获得表面平整、无气孔、表面硬度高且水平分布均匀的硬化层。激光熔凝处理后组织发生显著变化,高铬铸钢中的网状碳化物完全溶解,熔凝层内组织细化,组织形态由表及里依次为等轴晶—树枝晶—胞状晶,显微组织为奥氏体和颗粒状的M23C6碳化物。熔凝层内碳和合金元素的分布相对均匀,奥氏体组织受到固溶强化、位错强化及细晶强化的共同作用,硬度可达到473.1HV0.2。热影响区显微组织由隐晶马氏体、残余奥氏体和弥散碳化物组成,硬度达到759HV0.2。采用SYSWELD软件建立与实际光斑符合较好的热源模型,对激光熔凝过程中的温度场及应力场进行分析。结果表明,激光熔凝过程中,光斑中心瞬时加热速度可达3.2×104℃/s,瞬时冷却速度可达1.5×104℃/s。单道激光熔凝处理后,熔凝层承受拉应力,距光斑中心2.5mm处的热影响区,Mises应力达最大值713MPa。激光熔凝层由于组织具有较高的强韧性,未发生开裂,热影响区的组织韧性较差,且该区残余拉应力较大,沿碳化物与基体界面产生裂纹。预热和搭接处理可有效降低熔凝层残余拉应力和裂纹敏感性,预热150℃保温1h可预防热影响区开裂。激光熔凝层在低于400℃回火后,硬度基本保持在430HV0.2左右,低于未经回火处理的激光熔凝层硬度(473.1HV0.2)。回火过程中二次硬化始于450℃,此时熔凝层内仍存在孪晶及位错亚结构,细小M23C6碳化物的析出及少量马氏体的生成使熔凝层硬度略有增加(456HV0.2)。560℃回火后由于二次碳化物的析出、大量马氏体的生成及位错强化的共同作用,熔凝层硬度高达672HV0.2。经650℃回火基体完全转变为铁素体,析出的二次碳化物聚集长大、呈网状分布,硬度降低至400HV0.2。高温氧化试验表明,激光熔凝前后高铬铸钢在650℃时氧化缓慢,氧化动力学曲线近似呈对数规律,800℃时氧化速率剧增,氧化动力学曲线遵循抛物线规律。高铬铸钢基体的氧化膜由Fe和Fe2O3组成,高温下高铬铸钢氧化核心在基体与碳化物界面处优先生长,发生不均匀氧化,基体氧化严重,800℃时基体氧化膜开裂。激光熔凝层表面氧化膜由Fe、Fe2O3和(Fe0.6Cr0.4)2O3组成,由于熔凝层的组织细小,氧化初期表面近似均匀氧化,随后通过扩散控制氧化膜逐渐增厚。与未处理试样相比,激光熔凝试样的氧化膜较厚。高温磨损试验表明,激光熔凝处理后560℃和650℃的耐高温磨损性能明显提高,800℃时试样增重但增重量小于未处理试样。高铬铸钢560℃时发生磨粒磨损,磨损面出现大量细小的犁沟和碳化物颗粒。650℃时以粘着磨损为主,并伴随微观犁削。温度升高至800℃时,粘着严重,磨损面存在较深犁沟。激光熔凝处理后熔凝层具有较高的强韧性,560℃和650℃时的耐磨性明显提高,560℃的磨损机制为轻微的磨粒磨损,650℃时发生粘着磨损。激光熔凝试样高温下表面发生均匀氧化而形成连续致密的氧化膜,有效降低了激光熔凝层800℃时的粘着磨损倾向。更多还原

【Abstract】 High chrome cast steels are used successfully for the production of hot rollers. These steels possess a high mechanical strength at the working temperature and a high wear resistance. However, the primary carbides along grain boundaries constitute a favorable propagation path for the mechanical and thermal fatigue cracks. Moreover, during rolling the poor cooperation of eutectic carbides and matrix cause oxidation wear, deteriorating the high-temperature wear resistance. In this paper laser surface melting (LSM) were performed on the surface of high chrome cast steels, and the microstructure of the melted layer was analysed as well as the influence of overlapping parameter on the structure and property. High-temperature oxidation, wear resistance and the internal relationship were studied.High chrome cast steel is composed of tempered martensite and primary eutectic M7C3 carbides. The surface layer obtained after LSM with a power of 2.7kW and a traverse speed of 300mm/min was relatively smooth, morphological homogenous without presence of porosity. The results after electrochemical test show that for high chrome cast steel, corrosion attack at the phase boundaries between the tempering martensite matrix and the carbides was observed. Moreover, LSM leads to the enhancement in the corrosion resistance due to the combined effects of dissolution and refinement of large carbides and the increase of the alloying elements in the ultrafine solid solution of austenite. Using the overlapped ratio of 33.3% gives a more uniform hardened-depth, and uniform corrosion was observed in the laser-melted steel.After LSM the structure of high chrome cast steel changed dramatically, the eutectic carbides were completely dissoluted, and the microstructure in the melted layer is obviously refined. The bottom and the central region show cellular/dendritic structures while the upper region consists of equiaxed dendrites.The microstructure is austenite and the interdendritic carbides of type M23C6, and the hardness of the austenite can reach 473.1HV0.2 due to the combined effects of solid solution, dislocations and ultrafine grains. The HAZ consists of cryptyo-crytal martensite, retained austenite and dispersed carbides, while the microhardness of the HAZ is higher than that of the substrate, and reaches 759HV0.2. The laser overlapped zone is divided into the overlapped melted zone and the overlapped HAZ, and the overlapped melted zone is composed of austenitic dendrites while the structure at the overlapped HAZ is same as that of HAZ by the single track.Heat resource model, which is agreement with the real laser spot, was built by SYSWELD software, and the temperature and stress fields were studied by numerical simulation. The results show that during LSM the heating rate and the cooling rate at the center of the laser spot can reach 3.2×104℃/s and 1.5×104℃/s, respectively. After the single-track LSM, the laser melted layer bears tensile stress, and at the HAZ which is 2.5mm away from the center of the beam spot, the Mises stress reaches the maximum value of 713MPa, resulting in the high cracking susceptibiltiy. The structure in the laser melted layer possesses high strength and ductility without cracks. However, the cracks initiate along the interface between the carbides and the matrix because of the tensile stress at the HAZ as well as the low ductility of the structures. Moreover, overlapping can effectively decrease the residual stress and the cracking sensitivity. Testing proved that preheating at 150℃for 1h can avoid cracking at the HAZ.The hardness of the laser melted layer changes little and keeps at about 430HV0.2 when tempering below 400℃, which is slightly lower than that of the untemperte laser melted layer. The secondary hardening phenomenon appears, beginning at 450℃, at this time a large amount of the dislocation and slip bands substructures still exist in the austenite tempered at 450℃, and the precipitation of fine M23C6 carbides and the formation of martensite contribute to the slight increase in the hardness. When tempering at 560℃, the secondary hardening resulted simultaneously from the martensite phase transformation and the precipitation of secondary carbides as well as the dislocation strengthening within a refined microstructure. The matrix transformed to ferrite completely after tempering at 650℃, and the decrease of hardness could be caused by the coagulation of carbides.High-temperature oxidation at 650℃of high chrome cast steel before and after LSM is slow, following an oxidation kinetic with a logarithmic trend, while the oxidation increases at 800℃with a parabolic trend. The oxide scale of high chrome cast steel is composed of Fe and Fe2O3, oxidation nucleates at the carbide-matrix interface, even cracks at 800℃giving rise to an uneven oxide scale. For the laser melted steel, the oxide scales consist of Fe, Fe2O3 and (Fe0.6Cr0.4)2O3 after LSM. The oxide scale covers the laser melted layer evenly due to the refined microstructure and grows as a result of the electrical transport of electrons or ions across the oxide film. Compared with the as-received steel, the oxide scales of the laser melted specimens is thicker.The high temperature wear resistance of high chrome cast steel at 560℃and 650℃has been improver obviously by LSM and the weight gains at 800℃, but the gain in weight is lower than that of the as-received high chrome cast steel. At 560℃wear mechanism of high chrome cast steel is abrasive wear, while grooves and carbide particles appear on the worn surface. Adhesive wear dominates at 650℃with some scoring marks on the worn surface, while at 800℃sever adhesion occurs accompanied by some deep grooves. The wear resistance of the laser melted steel is improved at 560℃and 650℃resuting from the high strength and ductility and the wear mechanism is mainly slight abrasive wear which is adhesive wear at 650℃. The improved adhesion resistance and 800℃is attributed to the formation of the oxide protective layer formed on the worn surface due to even oxidation by way of tribochemical reactions.更多还原