17-7PH沉淀硬化不锈钢电渣重熔过程洁净度的变化

采用35 t电弧炉-AOD脱碳-LF精炼-模铸工艺制备了17-7PH沉淀硬化不锈钢自耗电极,并通过气体保护电渣炉重熔得到了2 t重的电渣锭。利用ASPEX扫描电镜分析了电渣重熔前后17-7PH钢中夹杂物数量、尺寸、成分的变化规律,并采用SEM-EDS进一步观察夹杂物的形貌及组成。研究结果发现,电渣重熔后,O含量由6.6×10^-6降至5.7×10^-6,N含量由200×10^-6降至180×10^-6。重熔前后夹杂物的类型没有变化,重熔后总的夹杂物数量大幅减少,特别是大颗粒夹杂物的数量明显减少、尺寸减小。电渣锭中总的夹杂物以AlN夹杂物为主,其尺寸较大、数量最多。为了提高17-7PH钢电渣锭的洁净度,应尽可能减少自耗电极中的N含量,以减少电渣重熔过程AlN夹杂物的生成量。     

海洋大气环境下17-7PH不锈钢的接触腐蚀研究

 为了研究17-7PH不锈钢在海洋大气环境下接触腐蚀的防护问题,将不同表面状态的17-7PH不锈钢板状试样在青岛团岛和海南万宁进行1年的大气暴晒试验,对其宏观腐蚀形貌对比,并测定其疲劳寿命,采用扫描电子显微镜(SEM)观察暴晒试样表面腐蚀形貌,用光学显微镜比较腐蚀坑深度,分析海洋大气腐蚀对17-7PH不锈钢疲劳性能的影响,最后得出了17-7PH不锈钢的腐蚀防护措施。结果表明：在青岛暴晒的不涂漆17-7PH不锈钢试样表面色泽变暗,有均匀的细小点蚀,而海南的试样表面有大面积较均匀的褐色锈层,特别是17-7PH不锈钢与TC18钛合金连接处腐蚀较为集中,但腐蚀并没有降低其疲劳寿命;从暴晒试样的表面微观腐蚀形貌比较,无论涂漆与否,17-7PH不锈钢表面都有轻微腐蚀,但只局限于表层,点蚀不深,并且趋向均匀腐蚀;17-7PH不锈钢抗大气腐蚀性能很好,经钝化后可不涂漆直接在海洋大气环境中使用1年,而不会对其疲劳性能产生明显影响。     

Tensile Mechanical Behavior and Spall Response of a Selective Laser Melted 17-4 PH Stainless Steel

The tensile mechanical behavior and spall response of a selective laser melted (SLM) 17-4 precipitation-hardening (PH) stainless steel were studied comprehensively through tensile test, plate impact experiment and microstructure characterization in the present study. The results reveal a steel with significant strain rate dependence on the tensile mechanical behavior and spall response. As the strain rate increases, the tensile yield stress increases, but there is no monotonic variation trend for the peak stress; grain structure remains unchanged first and then becomes fine; high-angle grain boundaries (HAGBs) increase; the martensite phase decreases at first and then increases. There is a close correlation among impact velocity, strain rate, peak stress, Hugoniot elastic limit (HEL) and spall strength. Strain rate, peak stress and HEL increase, while spall strength remains almost constant with the increase of impact velocity. As impact velocity increases, grain structure becomes fine, HAGBs increase and the martensite phase increases. The significant phase transformation is responsible for the tensile mechanical behavior and spall response, and the temperature rise was calculated to analyze its effect on phase transformation. Whether the preferred orientations are along the building direction or tensile direction is dependent on strain rate. Tensile and spallation specimens exhibit the ductile fracture mode and the damage originates from voids. It is interesting that the voids always tend to nucleate at melt pool boundaries. A spall damage evolution model is illustrated to describe the damage process.

     

N含量对17-4PH马氏体时效不锈钢铁素体相和性能的影响

试验用17-4PH不锈钢(/%:0.02～0.03C,0.40～0.54Si,0.41～0.60Mn,0.018～0.026P,0.001S,15. 56～15. 75Cr,4. 31～4. 50Ni,3. 21～3.40Cu,0. 20～0. 25Nb,0. 013～0. 067N)采用20 t EAF+LF+VOD+ESR的工艺冶炼,锻造成Φ150 mm棒材。试验结果表明:钢中N含量由0.013%增加至0.067%时,钢中铁素体含量由0.5%增至6%,钢的强度、硬度升高,塑性降低。当N含量控制在0.020%～0.045%,钢中铁素体含量为1%～3%,经1040℃ 1h水冷+480℃  2 h空冷后,钢的力学性能合格;固溶态钢的HB硬度值≤325,优化了该材料的加工性能。     

Effects of the Process Parameters on the Microstructure and Properties of Nitrided 17-4PH Stainless Steel

The effects of process parameters on the microstructure, microhardness, and dry-sliding wear behavior of plasma nitrided 17-4PH stainless steel were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and wear testing. The results show that a wear-resistant nitrided layer was formed on the surface of direct current plasma nitrided 17-4PH martensitic stainless steel. The microstructure and thickness of the nitrided layer is dependent on the treatment temperature rather than process pressure. XRD indicated that a single α _N phase was formed during nitriding at 623 K (350 °C). When the temperature increased, the α _N phase disappeared and CrN transformed in the nitrided layer. The hardness measurement demonstrated that the hardness of the stainless substrate steel increased from 320 HV_0.1 in the untreated condition increasing to about 1275HV_0.1 after nitriding 623 K (350 °C)/600 pa/4 hours. The extremely high values of the microhardness achieved by the great misfit-induced stress fields associated with the plenty of dislocation group and stacking fault. Dry-sliding wear resistance was improved by DC plasma nitriding. The best wear-resistance performance of a nitrided sample was obtained after nitriding at 673 K (350 °C), when the single α _N-phase was produced and there were no CrN precipitates in the nitrided layer.     

固溶处理和时效对17-4PH沉淀硬化不锈钢组织性能的影响

试验得出O．053C-16．0Cr-4．5Ni-3．1Cu(17-4PH)沉淀硬化不锈钢1040℃1h油冷，固溶处理，470～490℃时效2h的组织和性能最佳，硬度可达HRC50。     

Microstructure and Wear Behaviour of 17-4 Precipitation Hardening Stainless Steel with Various Ti Content

Powder Metallurgy and Metal Ceramics - In this study, the wear behaviour of aged 17-4 PH SS (precipitation hardening stainless steel) that contains 0.5, 1, 1.5, and 2% of Ti was examined. The mixed...     

EAF-VOD-2.5 t ESR-锻造流程生产Φ350 mm 17-4PH不锈钢

采用EAF -VOD工艺冶炼17-4PH沉淀硬化不锈钢(%≤0.04C、16.20～16.50Cr、4.50～4.70Ni、3.30～3.40Cu、0.25～0.40Nb),铁模下铸Φ320 mm电极,并电渣重熔(ESR)成2.5 t(Φ510～550 mm)锭,经800 t液压机或5 t蒸汽锻锤生产17-4PH钢Φ350 mm大规格锻材成品.工艺实践表明,控制停锻温度(蒸气锤980 ℃;液压机1050 ℃),将锻成的Φ350 mm 17-4PH钢成品材返回至室式加热炉(1130～1150 ℃)均热,缓冷至(1050±10)℃均热后,炉冷至420 ℃,并及时在650 ℃退火,有效地消除了该钢锻后炸裂现象.     

Effects of the Treating Time on Microstructure and Erosion Corrosion Behavior of Salt-Bath-Nitrided 17-4PH Stainless Steel

The effects of salt-bath nitriding time on the microstructure, microhardness, and erosion-corrosion behavior of nitrided 17-4PH stainless steel at 703 K (430 °C) were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and erosion-corrosion testing. The experimental results revealed that the microstructure and phase constituents of the nitrided surface alloy are highly process condition dependent. When 17-4PH stainless steel was subjected to complex salt-bathing nitriding, the main phase of the nitrided layer was expanded martensite (α`), expanded austenite (S), CrN, Fe₄N, and Fe₂N. The thickness of nitrided layers increased with the treating time. The salt-bath nitriding improves effectively the surface hardness. The maximum values measured from the treated surface are observed to be 1100 HV_0.1 for 40 hours approximately, which is about 3.5 times as hard as the untreated material (309 HV_0.1). Low-temperature nitriding can improve the erosion-corrosion resistance against two-phase flow. The sample nitrided for 4 hours has the best corrosion resistance.     

Precipitation Kinetics During Post-heat Treatment of an Additively Manufactured Ferritic Stainless Steel

Metallurgical and Materials Transactions A - The microstructure response of laser-powder bed fusion (L-PBF)-processed ferritic stainless steel (AISI 441) during post-heat treatments is studied in...     

Diffusion Bonding of 17-4 Precipitation Hardening Stainless Steel to Ti Alloy With and Without Ni Alloy Interlayer: Interface Microstructure and Mechanical Properties

In the present study, the diffusion bonding of 17-4 precipitation hardening stainless steel to Ti alloy with and without nickel alloy as intermediate material was carried out in the temperature range of 1073 K to 1223 K (800 °C to 950 °C) in steps of 298 K (25 °C) for 60 minutes in vacuum. The effects of bonding temperature on interfaces microstructures of bonded joint were analyzed by light optical and scanning electron microscopy. In the case of directly bonded stainless steel and titanium alloy, the layerwise α-Fe + χ, χ, FeTi + λ, FeTi + β-Ti phase, and phase mixture were observed at the bond interface. However, when nickel alloy was used as an interlayer, the interfaces indicate that Ni₃Ti, NiTi, and NiTi₂ are formed at the nickel alloy-titanium alloy interface and the PHSS-nickel alloy interface is free from intermetallics up to 1148 K (875 °C) and above this temperature, intermetallics were formed. The irregular-shaped particles of Fe₅Cr₃₅Ni₄₀Ti₁₅ have been observed within the Ni₃Ti intermetallic layer. The joint tensile and shear strength were measured; a maximum tensile strength of ~477 MPa and shear strength of ~356.9 MPa along with ~4.2 pct elongation were obtained for the direct bonded joint when processed at 1173 K (900 °C). However, when nickel base alloy was used as an interlayer in the same materials at the bonding temperature of 1148 K (875 °C), the bond tensile and shear strengths increase to ~523.6 and ~389.6 MPa, respectively, along with 6.2 pct elongation.     

Precipitation Kinetics in a Nb-stabilized Ferritic Stainless Steel

The precipitation occurring in a Nb-stabilized ferritic stainless steel, containing initially Nb(C, N) carbonitrides and Fe₃Nb₃X precipitates, was investigated during aging treatments performed between 923 K and 1163 K (650 °C and 890 °C) by combining different techniques, (thermoelectric power (TEP), scanning/transmission electron microscopy (SEM/TEM), and atom probe tomography (APT)), in order to determine the precipitation kinetics, the nature and morphology of the newly formed precipitates as well as the chemistry of the initial Fe₃Nb₃X precipitates, where X stands for C or N. The following composition was proposed for these precipitates: (Fe_0.81 Cr_0.19)₃ (Nb_0.85 Si_0.08 Mo_0.07)₃ (N_0.8 C_0.2), highlighting the simultaneous presence of N and C in the precipitates. With regard to the precipitation in the investigated temperature range, two main phenomena, associated with a hardness decrease, were clearly identified: (i) the precipitation of Fe₂Nb precipitates from the niobium initially present in solution or coming from the progressive dissolution of the Fe₃Nb₃X precipitates and (ii) the precipitation of the χ-phase at grain boundaries for longer aging times. From the TEP kinetics, a time–temperature–precipitation diagram has been proposed.     

Effect of Treatment Time on the Microstructure of Austenitic Stainless Steel During Low-Temperature Liquid Nitrocarburizing

The effect of treatment time on the microstructure of AISI 304 austenitic stainless steel during liquid nitrocarburizing (LNC) at 703 K (430 °C) was investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Experimental results revealed that the modified layer was covered with the alloy surface and the modified layer depth increased extensively from 2 to 33.4 μm with increasing treatment time. SEM and XRD showed that when the 304 stainless steel sample was subjected to LNC at 703 K (430 °C) for less than 4 hours, the main phase of the modified layer was expanded austenite. When the treatment time was prolonged to 8 hours, the abundant expanded austenite was formed and it partially transformed into CrN and ferrite subsequently. With the increased treatment time, more and more CrN precipitate transformed in the overwhelming majority zone in the form of a typical dendritic structure in the nearby outer part treated for 40 hours. Still there was a single-phase layer of the expanded austenite between the CrN part and the inner substrate. TEM showed the expanded austenite decomposition into the CrN and ferrite after longtime treatment even at low temperature.     

Effect of Aging on Hardening Behavior of 15-5 PH Stainless Steel

Microstructure transformation and aging hardening behavior of 15-5PH stainless steel were studied by optical microscopy(OM),X-ray diffraction(XRD)and transmission electron microscopy(TEM).The results showed that the 15-5PH stainless steel consists of NbC precipitates and lath matensite with a high dislocation density after solution treatment.With increasing aging temperature and aging time,the martensitic laths were resolved gradually.Meanwhile,the nanometric-sized Cu precipitates gradually coarsened and lost their coherency with the martensite matrix,which exhibited an elliptical shape finally.Fine Cu precipitates can lead to significant dispersion hardening effect,while the coarsened Cu precipitates have no contribution to strengthening.The reversed austenite was observed in the specimens aged at 550 ℃ and above;moreover,the amount of reversed austenite increased as aging temperature increased.The precipitation hardening behavior of 15-5PH stainless steel may depend on the balance between the softening caused by the formation of reversed austenite and the hardening caused by the precipitation of copper.