The microstructure and properties of 17-4PH martensitic precipitation hardening stainless steel modified by plasma nitrocarburizing

17-4PH martensitic precipitation hardening stainless steel was plasma nitrocarburized at 430 °C and 460 °C for 8 h. The nitrocarburized layers were characterized by optical microscope, scanning electron microscope, X-ray diffractometer, microhardness tests, pin-on-disc tribometer and the anodic polarization method in a 3.5% NaCl solution. The results show that the microstructure of plasma nitrocarburized layer is characterized by a compound layer with no evident diffusion zone. The phases in the 430 °C treated layer are mainly of γ′-Fe₄N, nitrogen and carbon expanded martensite (α′_N), and some incipient CrN phases. When the temperature increases up to 460 °C, there is no evidence of α′_N phase. The processes of bulk precipitation hardening and surface treatment by plasma nitrocarburizing can be successfully combined in a single-step process on this steel. The hardness of modified layer can reach up to 1186HV, which is 3 times higher than that of untreated steel. The wear and corrosion resistance of the specimens can be apparently improved by plasma nitrocarburizing. The 460 °C/8 h treated specimen has the best wear and corrosion resistance in the present test conditions.     

The corrosion and corrosion-wear behaviour of plasma nitrided 17-4PH precipitation hardening stainless steel

Plasma surface nitriding of 17-4 PH martensitic precipitation hardening stainless steels was conducted at 350 °C, 420 °C and 500 °C for 10 h using a DC plasma nitriding unit, and the surface properties of the plasma surface engineered samples were systematically evaluated. Experimental results have shown that the surface properties of the plasma nitrided layers in terms of hardness, wear resistance, corrosion behaviour and corrosion-wear resistance are highly process condition dependent, and it is feasible to provide considerable improvement in wear, corrosion and corrosion-wear resistance of 17-4PH steel using optimised plasma treatment conditions. All three treatments can effectively improve the surface hardness and the sliding wear resistance under unlubricated conditions; high temperature (420 °C and 500 °C) treated materials revealed improved corrosion and corrosion-wear properties due to the formation of surface compound layers.     

Effects of Temperature on Microstructure and Wear of Salt Bath Nitrided 17-4PH Stainless Steel

Salt bath nitriding of 17-4 PH martensitic precipitation hardening stainless steels was conducted at 610, 630, and 650?°C for 2?h using a complex salt bath heat-treatment, and the properties of the nitrided surface were systematically evaluated. 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 (????), expanded austenite (??_N), CrN, Fe₄N, and (Fe,Cr) _x O _y . In the sample nitrided above 610?°C, the expanded martensite transformed into expanded austenite. But in the sample nitrided at 650?°C, the expanded austenite decomposed into ??_N and CrN. The decomposed ??_N then disassembled into CrN and alpha again. The nitrided layer depth thickened intensively with the increasing nitriding temperature. The activation energy of nitriding in this salt bath was 125?±?5?kJ/mol.     

SLM成形17-4PH不锈钢微观组织与动态断裂性能

通过激光选区熔化(selective laser melting, SLM)技术制备了17-4PH不锈钢,采用电子背散射衍射(electron backscattered diffraction, EBSD)和透射电子显微镜(transmission electron microscope, TEM)等方法对沉积态和固溶态试样微观组织结构进行了分析.通过示波冲击试验确定了裂纹萌生扩展的特征阶段和动态裂纹扩展阻力曲线(J-R曲线),研究了微观组织与动态断裂性能之间的关系.结果表明,沉积态试样主要由<100>择优且沿增材方向拉长的δ铁素体柱状晶、取向随机的细小马氏体,以及少量奥氏体组成,不同截面具有显著的组织各向异性;大尺寸δ铁素体柱状晶与细小晶粒的结合面作为薄弱环节,使其脆性增加,J-R曲线的撕裂模量较低,以准解理方式断裂.固溶热处理明显弱化组织各向异性,微观组织由尺寸细小、均匀的马氏体组成,其冲击吸收能量提升1倍,动态断裂韧性优良,属于韧性断裂.大尺寸δ铁素体柱状晶与周围细小马氏体晶粒界面结合较弱是沉积态17-4PH不锈钢动态断裂性能较差的主要原因.     

Improvement of corrosion and wear resistances of AISI 420 martensitic stainless steel using plasma nitriding at low temperature

The influence of low temperature plasma nitriding on the wear and corrosion resistance of AISI 420 martensitic stainless steel was investigated. Plasma nitriding experiments were carried out with DC-pulsed plasma in 25% N₂ + 75% H₂ atmosphere at 350 °C, 450 °C and 550 °C for 15 h. The composition, microstructure and hardness of the nitrided samples were examined. The wear resistances of plasma nitrided samples were determined with a ball-on-disc wear tester. The corrosion behaviors of plasma nitrided AISI420 stainless steel were evaluated using anodic polarization tests and salt fog spray tests in the simulated industrial environment.The results show that plasma nitriding produces a relatively thick nitrided layer consisting of a compound layer and an adjacent nitrogen diffusion layer on the AISI 420 stainless steel surface. Plasma nitriding not only increases the surface hardness but also improves the wear resistance of the martensitic stainless steel. Furthermore, the anti-wear property of the steel nitrided at 350 °C is much more excellent than that at 550 °C. In addition, the corrosion resistance of AISI420 martensitic stainless steel is considerably improved by 350 °C low temperature plasma nitriding. The improved corrosion resistance is considered to be related to the combined effect of the solid solution of Cr and the high chemical stable phases of ?-Fe₃N and α_N formed on the martensitic stainless steel surface during 350 °C low temperature plasma nitriding. However, plasma nitriding carried out at 450 °C or 550 °C reduces the corrosion resistance of samples, because of the formation of CrN and leading to the depletion of Cr in the solid solution phase of the nitrided layer.     

Effect of DC plasma nitriding temperature on microstructure and dry-sliding wear properties of 316L stainless steel

A wear resistant nitrided layer was formed on 316L austenitic stainless steel substrate by DC plasma nitriding (DCPN). The structural phases, micro-hardness and dry-sliding wear behavior of the nitrided layer were investigated by optical microscopy (OM), X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDS), micro-hardness tester and ring-on-block wear tester. The results show that a single expanded austenite layer (S-phase) and a single CrN nitride layer were formed at 400 °C and 480 °C, respectively. In addition, the S-phase layers formed on the samples exhibited the best dry-sliding wear resistance under ring-on-block contact configuration test. Wear of the untreated 316L was sever and characterized by strong adhesion, abrasion and oxidation mechanism, whilst wear of the DCPN-treated 316L was mild and dominated by plastic deformation, slight abrasion and frictional polishing.     

淬火-配分处理对二次淬火时钢中残留奥氏体分解转变的影响

对0.26C-1.72Si-1.56Mn钢进行了不同碳配分时间的淬火-配分(Q-P)处理,并研究了其组织,特别是二次淬火中奥氏体的分解转变。结果表明:Q-P处理后都形成了板条马氏体+二次淬火组织,且二次淬火组织中都存在孪晶马氏体;碳配分时间在10~300 s范围内,Q-P处理后残留奥氏体中的C含量均高于1.0wt%,残留奥氏体的含量不低于11%(体积分数),有利于钢韧性的改善;初次淬火后未转变奥氏体的形态和尺寸是影响其稳定性的关键因素,初次马氏体板条界膜状奥氏体容易形成残留奥氏体;相对于块状未转变奥氏体,条状未转变奥氏体容易形成二次淬火马氏体及片状残留奥氏体。     

Grain refinement and mechanical properties of asymmetrically rolled low carbon steel

The formation of fine ferrite grains by the asymmetric rolling of low carbon steel and their mechanical properties were studied. Super-cooled low carbon austenite was deformed by asymmetric rolling at 750 °C with a roll size ratio of 1.5 and immediately cooled at various cooling rates ranging from 3 °C/s to 15 °C/s. Fine ferrite grains (∼2 μm) were formed after asymmetric rolling, preferentially at the prior austenite grain boundaries. The volume fraction of the fine ferrite grains increased with increasing rolling reduction. A ferrite plus pearlite microstructure was obtained at smaller strains and slower cooling rates. However, after heavy deformation, a fine ferrite grain structure with carbide particles dispersed at the ferrite grain boundaries was obtained and the pearlite structure was not observed even after very slow cooling, which implies that most of the ferrite grains were formed dynamically, i.e. during deformation. The yield strength of the asymmetrically rolled steel plates increased with increasing deformation; however, the yield ratio also increased with increasing rolling reduction. The best combination of strength and yield ratio was obtained by using a low level of deformation and a high cooling rate, in which case a portion of the untransformed austenite transformed to martensite.     

Microstructure Characterization and Corrosion Properties of Nitrocarburized AISI 4140 Low Alloy Steel

Plasma nitrocarburizing treatments of AISI 4140 low alloy steel have been carried out in a gas mixture of 85% N₂-12% H₂-3% CO₂. All treatments were performed for 5 h at a chamber pressure of 4 mbar. Different treatment temperatures varying from 520 to 620 °C have been used to investigate the effect of treatment temperature on the corrosion and hardness properties and also microstructure of the plasma nitrocarburized steel. Scanning electron and optical microscopy, x-ray diffraction, microhardness measurement, and potentiodynamic polarization technique in 3.5% NaCl solution were used to study the treated surfaces. The results revealed that plasma nitrocarburizing at temperatures below 570 °C can readily produce a monophase ε compound layer. The compound layer formed at 620 °C is composed of two sub-layers and is supported by an austenite zone followed by the diffusion layer. The thickest diffusion layer was related to the sample treated at 620 °C. Microhardness results showed a reduction of surface hardness with increasing the treatment temperature from 520 to 620 °C. It has also been found that with increasing treatment temperature from 520 to 545 °C the corrosion resistance increases up to a maximum and then decreases with further increasing treatment temperature from 545 to 620 °C.     

预相变马氏体对超级贝氏体钢组织热稳定性的影响

为明确超级贝氏体组织失稳机制以及探索提高超级贝氏体钢中残余奥氏体热稳定性的方法,通过预相变马氏体工艺,即在等温贝氏体相变前引入预相变马氏体,制备了中碳超级贝氏体钢。对比分析了回火前后中碳超级贝氏体钢显微组织和力学性能的变化,研究了预相变马氏体对中碳超级贝氏体钢中贝氏体组织及残余奥氏体热稳定性的影响。结果表明：预相变马氏体的存在能够细化贝氏体铁素体板条,提高残余奥氏体含量和热稳定性。预相变马氏体的引入及其对超级贝氏体组织的细化作用使得试验钢的屈服强度超过1 000 MPa,伸长率大于20%;300～600℃回火1 h后,高碳薄膜状残余奥氏体首先发生分解,形成细小的碳化物,然后贝氏体铁素体板条发生回复和再结晶,形成沿原板条方向的铁素体晶粒;600℃回火后试验钢的屈服强度仍与回火前相当,主要是预相变马氏体周围的薄膜状残余奥氏体未发生明显分解,能够抑制相邻贝氏体铁素体板条的回复。     

Microstructure and mechanical properties of 17-4PH steel plasma nitrocarburized with and without rare earths addition

17-4PH stainless steel was plasma nitrocarburized at 500 °C with and without rare earths (RE) addition. The nitrocarburized layers were characterized by optical microscope, scanning electron microscope equipped with an energy dispersive X-ray analyzer, X-ray diffraction, hardness tests and pin-on-disc tribometer. The results show that rare earths atoms can diffuse into 17-4PH steel surface and change the microstructure of the nitrocarburized layer. The incorporation of RE elements increases the layer thickness and the hardness of the nitrocarburized layer up to 29% and 70–120 HV, respectively. The friction coefficients and wear rates of the nitrocarburized specimens are apparently lower than that of un-nitrocarburized one. The wear mechanisms of steel specimen plasma nitrocarburized with and without RE addition are different mainly due to the differences in the microstructure, the phase proportion and the hardness of the modified layer.     

Corrosion properties of active screen plasma nitrided 316 austenitic stainless steel

AISI 316 austenitic stainless steel has been plasma nitrided using the active screen plasma nitriding (ASPN) technique. Corrosion properties of the untreated and AS plasma nitrided 316 steel have been evaluated using various techniques, including qualitative evaluation after etching in 50%HCl + 25%HNO₃ + 25%H₂O, weight loss measurement after immersion in 10% HCl, and anodic polarisation tests in 3.5% NaCl solution. The results showed that the untreated 316 stainless steel suffered severe localised pitting and crevice corrosion under the testing conditions. AS plasma nitriding at low temperature (420 °C) produced a single phase nitrided layer of nitrogen expanded austenite (S-phase), which considerably improved the corrosion properties of the 316 austenitic stainless steel. In contrast, AS plasma nitriding at a high temperature (500 °C) resulted in chromium nitride precipitation so that the bulk of the nitrided case had very poor corrosion resistance. However, a thin deposition layer on top of the nitrided case, which seems to be unique to AS plasma nitriding, could have alleviated the corrosion attack of the higher temperature nitrided 316 steel.     

Phase transformations in low-temperature chromized 0.45 wt.% C plain carbon steel

The phase transformations occurring in a 0.45 wt.% C plain steel subjected to plasma nitriding at 540-560 °C for 5.5 h, followed by a salt bath thermoreactive deposition and diffusion (TRD) chromizing process at 500 °C or 550 °C (a process referred to as low-temperature chromizing or duplex chromizing) was investigated by means of optical microscopy(OM), scanning electron microscopy(SEM), X-ray energy dispersive spectroscopy(EDS), and X-ray diffraction. It was found that a CrN compound layer with an average thickness of 7.4 μm and an average micro-hardness of 1476 HV_0.01 was formed in the prior plasma nitrided compound layer by low-temperature chromizing at 550 °C for 6 h. The chromized coating as a whole was found consisting of three sub-layers, namely the outer CrN layer, the intermediate diffusional layer, and the inner residual nitrided compound layer, all formed in the prior nitrided compound layer, and with the inner sub-layer vanishing by prolonging the chromizing time. The intermediate diffusional layer formed at the initial stages of TRD was seen “black” under OM (hence is called “black zone”), and found consisting of α-Fe as a major phase. The self-exhaustion of the “black zone” promoted the chromium atom diffusion deeper into the substrates. The transformation paths involved in the decomposition of the prior nitrided compound layer was likely to be ε-Fe_2-3N → γ′-Fe₄N → α-Fe; and the high hardness of the chromized coating was attributed to a large amount of nano-sized and evenly distributed CrN grains generated in the compound layer.     

Influence of Isothermal Bainite Transformation Time on Microstructure and Mechanical Properties of Hot-Dip Galvanized TRIP Steel

The influence of isothermal bainitic transformation (IBT) time on microstructure and mechanical properties of hot-dip galvanized TRIP steel with 0.20C-1.50Mn-1.2Al-0.26Si was investigated using optical microscopy, x-ray diffraction (XRD), Transmission Electron Microscope (TEM), dilatometry, and mechanical testing. This steel has high tensile strength of over 780 MPa with elongation more than 22%. The microstructure of the steel mainly consisted of ferrite, bainite, retained austenite, and martensite. The metastable austenite remaining after bainitic transformation will be transformed into martensite at the final cooling stage. The IBT time affects retained austenite content. When the IBT increased from 10 to 60 s, the amount of retained austenite increased from 9.40 to 15.42%, correspondingly. The IBT time also affects the strain hardening behavior. The n value characteristics of samples for IBT time from 10 to 30 s are similar to those of DP steel; however, the n value of specimen with IBT of 60 s shows features typical of TRIP steel.     

Low-temperature formation and oxidation resistance of ultrafine aluminide coatings on Ni-base superalloy

Ultrafine aluminide coatings were successfully produced on Ni-18Fe-17Cr superalloy at 540-600 °C in a modified pack-aluminizing process. Repeated ball-impacts accelerated the formation of the aluminide coatings by a surface refining process, resulting in atomic diffusion occurring at a relatively low temperature. The effects of the operation temperature and the treatment duration on the formation of the coatings have been investigated. The coatings possessed a two-layer structure. The top layer, approximately 5 µm in thickness, exhibited equiaxial coarse grains and was dominated by NiAl₃, with small amounts of Fe₂Al₅ and CrAl₅. The bottom layer showed high density, homogeneous, ultrafine grains with diameters approximately 30-50 nm. High-temperature oxidation tests were carried out at 1000 °C. The oxidation kinetics and microstructure of the oxide scale were studied. The experimental results indicated that the coatings greatly enhanced the high-temperature oxidation resistance of Ni-18Fe-17Cr superalloy.     

采用金相法对17-7PH不锈钢钢丝表层残留奥氏体深度测试

AMS5678G材料规范要求采用金相法对17-7PH不锈钢钢丝表层残留奥氏体深度进行测试,针对测试中存在的试样制备和残留奥氏体层深度界定问题,对试样夹的选择、侵蚀方式与侵蚀时间、残留奥氏体层深度界定进行了试验。结果表明,为避免残留奥氏体显示时化学浸蚀电位的影响,宜采用塑料试样夹进行镶嵌;使用Fry’s试剂以擦拭的方式,擦拭5～10 s显示组织较合适;当钢丝表层存在残留奥氏体层时,其深度界定为致密的残留奥氏体层深度,得到材料规范归口单位认可。     

Formation and microstructural characterisation of S-phase layers in Ni-free austenitic stainless steels by low-temperature plasma surface alloying

The feasibility of generating S-phase surface layers in nickel-free austenitic stainless steels by plasma surface alloying with nitrogen (at 430 °C), carbon (at 430 °C and 500 °C) and both carbon and nitrogen (at 430 °C) has been investigated. The structure, microstructure and composition of the plasma-alloyed surfaces were characterised by X-ray Diffraction (XRD), microscopy, Glow Discharge Optical Emission Spectroscopy (GDOES) and Transmission Electron Microscopy (TEM). The experimental results have demonstrated for the first time that the S-phase can be produced in the surface of nickel-free austenitic stainless steel by low-temperature plasma surface alloying. TEM analysis has revealed that when alloyed with carbon no precipitates can be found within the carbon-rich S-phase layer; however, when alloyed with nitrogen or both carbon and nitrogen some nitride precipitates (Mn₃N₂ and Cr₂N) were found within the nitrogen-rich S-phase layer. Based on experimental results, the response of Ni-free austenitic stainless steel to plasma surface alloying has been compared to the Ni-containing counterpart, and the role of nickel in the formation of S-phase in austenitic stainless steels has been discussed.     

17-4PH不锈钢480 ℃时效组织的动态演变规律

将17-4PH不锈钢锻棒固溶处理后油冷,然后选择在最佳的时效温度480 ℃时效保温0～5 h后空冷。通过光学显微镜(OM)、超景深显微镜、XRD、显微硬度仪等测试方法观察固溶、时效过程的组织演变和分析其沉淀硬化机理;采用电阻仪间接测试ε-Cu相动态时效析出过程对电阻的影响;并利用摩擦磨损试验机测试其耐磨性能。研究发现:17-4PH不锈钢固溶和时效过程没有残留奥氏体和逆转变奥氏体出现,热处理后出现板条状和块状两种马氏体形态,板条状马氏体硬度高于块状马氏体,随着时效时间的延长,两种马氏体硬度同步上升,时效析出明显提高了固溶态组织的硬度;时效2.0～2.5 h附近强化效果和耐磨性能最弱,可能与ε-Cu 相长大及与位错交互作用有关;硬度随时效时间的变化趋势与电阻正好相反。     

高铬铸铁芯焊条堆焊层组织分析

基于焊芯过渡合金元素的技术思路,研制了高铬合金铸铁同质堆焊焊条．分析了不同药皮堆焊焊条的堆焊层组织及性能,定量表征了合金元素的过渡系数．结果表明,通过焊芯过渡合金元素的高铬合金铸铁堆焊焊条可获得组织和性能均匀的堆焊层．合金过渡系数高于85％．碱性药皮堆焊焊条堆焊层为亚共晶成分高铬合金铸铁,组织由奥氏体γ＋马氏体M＋碳化物Cr7C3组成．堆焊层硬度为44．5～56．5HRC．碱性石墨化型药皮堆焊焊条堆焊层组织由初生碳化物Cr7C3＋马氏体M＋碳化物Fe7C3＋少量石墨G组成,堆焊层硬度可达59～67HRC．