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Welding of nickel free high nitrogen stainless steel: Microstructure and mechanical properties

Defence Technology（防务技术） 2017年第2期13卷 59-71页

作者： Raffi Mohammed G.Madhusudhan Reddy k.srinivasa rao Department of Metallurgical Engineering Andhra University Visakhapatnam India Defence Metallurgical Research Laboratory Hyderabad India

High nitrogen stainless steel(HNS) is a nickel free austenitic stainless steel that is used as a structural component in defence applications for manufacturing battle tanks as a replacement of the existing armour grade steel owing to its low cost, excellent mechanical properties and better corrosion *** fusion welding causes problems like nitrogen desorption, solidification cracking in weld zone, liquation cracking in heat affected zone, nitrogen induced porosity and poor mechanical *** above problems can be overcome by proper selection and procedure of joining process. In the present work, an attempt has been made to correlate the microstructural changes with mechanical properties of fusion and solid state welds of high nitrogen steel. Shielded metal arc welding(SMAW), gas tungsten arc welding(GTAW), electron beam welding(EBW) and friction stir welding(FSW) processes were used in the present work. Optical microscopy, scanning electron microscopy and electron backscatter diffraction were used to characterize microstructural changes. Hardness, tensile and bend tests were performed to evaluate the mechanical properties of welds. The results of the present investigation established that fully austenitic dendritic structure was found in welds of SMAW. Reverted austenite pools in the martensite matrix in weld zone and unmixed zones near the fusion boundary were observed in GTA welds. Discontinuous ferrite network in austenite matrix was observed in electron beam *** recrystallized austenite grain structure was observed in the nugget zone of friction stir *** mechanical properties are obtained in friction stir welds when compared to fusion welds. This is attributed to the refined microstructure consisting of equiaxed and homogenous austenite grains.

关键词： High nitrogen austenitic stainless steel(HNS) Shielded metal arc welding (SMAW) Gas tungsten arc welding (GTAW) Electron beam welding (EBW) Friction stir welding (FSW)

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Ballistic behavior of boron carbide reinforced AA7075 aluminium alloy using friction stir processing-An experimental study and analytical approach

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Defence Technology（防务技术） 2016年第1期12卷 25-31页

作者： I.SUDHAkAR G.MADHUSUDHAN REDDY k.srinivasa rao department of mechanical engineering mvgrceVizianagaramIndia defence metallurgical research laboratory HyderabadIndia department of metallurgical engineering andhra universityVisakhapatnamIndia

High strength-to-weight ratio of non-ferrous alloys, such as aluminium, magnesium and titanium alloys, are considered to be possible replacement of widely accepted steels in transportation and automobile sectors. Among these alloys, magnesium is self explosive and titanium is costlier, and aluminium is most likely to replace steels. Application of aluminium or its alloys is also thought of as an appropriate replacement in defence field, especially to enhance the easiness in mobility of combat vehicles while maintaining the same standard as that of conventional armour grade steels. Hence most of the investigations have been confined to aluminium or its alloys as base material and open an era of developing the newer composite materials to address the major limitation, i.e. tribological properties. The surface composites can be fabricated by incorporating the ceramic carbides like silicon carbide, carbides of transition metals and oxides of aluminium using surface modification techniques, such as high energy laser melt treatment, high energy electron beam irradiation and thermal spray process which are based on fusion route. These techniques yield the fusion related problems, such as interfacial reaction, pin holes, shrinkage cavities or voids and other casting related defects, and pave the way to need of an efficient technique which must be based on solid state. Recently developed friction stir processing technique was used in the present investigation for surface modification of AA7075 aluminum alloy, which is an alternative to steels. In the present investigation, 160 μm sized boron carbide powder was procured and was reduced to 60 μm and 30 μm using high energy ball mill. Subsequently these powders were used to fabricate the surface composites using friction stir *** performance testing as per the military standard(JIS.0108.01) was carried out. In the present work, an analytical method of predicting the ballistic behavior of surface composites was

关键词： AA7075 aluminium alloy Friction stir processing(FSP) Surface metal matrix composite(SMMC) Boron carbide(B4C) Ballistic performance Target

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Microstructure and corrosion behaviour of gas tungsten arc welds of maraging steel

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Defence Technology（防务技术） 2015年第1期11卷 48-55页

作者： G.MADHUSUDHAN REDDY k.srinivasa rao Defence Metallurgical Research Laboratory Department of Metallurgical Engineering Andhra University

Superior properties of maraging steels make them suitable for the fabrication of components used for military applications like missile covering, rocket motor casing and ship hulls. Welding is the main process for fabrication of these components, while the maraging steels can be fusion welded using gas tungsten arc welding(GTAW) process. All these fabricated components require longer storage life and a major problem in welds is susceptible to stress corrosion cracking(SCC). The present study is aimed at studying the SCC behaviour of MDN 250(18% Ni) steel and its welds with respect to microstructural changes. In the present study, 5.2 mm thick sheets made of MDN 250 steel in the solution annealed condition was welded using GTAW process. Post-weld heat treatments of direct ageing(480 C for 3 h), solutionizing(815 C for 1 h) followed by ageing and homogenizing(1150 C for 1 h) followed by ageing were carried out. A mixture of martensite and austenite was observed in the microstructure of the fusion zone of solutionized and direct aged welds and only martensite in as-welded condition. Homogenization and ageing treatment have eliminated reverted austenite and elemental segregation. Homogenized welds also exhibited a marginal improvement in the corrosion resistance compared to those in the as-welded, solutionized and aged condition. Constant load SCC test data clearly revealed that the failure time of homogenized weld is much longer compared to other post weld treatments, and the homogenization treatment is recommended to improve the SCC life of GTA welds of MDN 250 Maraging steel.

关键词： 18% Ni maraging steel Gas tungsten arc welding Post weld heat treatment Solutionising Ageing treatment Pitting corrosion Stress corrosioncracking (SCC)

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Microstructure and pitting corrosion resistance of AA2219 Ale Cu alloy friction stir welds e Effect of tool profile

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Defence Technology（防务技术） 2015年第2期11卷 123-131页

作者： Ch VENkATA rao G.MADHUSUDHAN REDDY k.srinivasa rao Department of Metallurgical Engineering Andhra University Defence Metallurgical Research Laboratory

AA2219 Ale Cu alloy is widely used in defence and aerospace applications due to required combination of high strength-to-weight ratio and *** of components used for defence always involves *** though the mechanical properties of the base metal are better,but the alloy suffers from poor mechanical and corrosion properties during fusion *** overcome the problems of fusion welding,friction stir welding(FSW) is recognized as an alternative solid state joining method aimed to improve the mechanical and corrosion *** profile is one of the important variables which affect the performance of the friction stir *** the present work the effect of tool profile on the microstructure and pitting corrosion of AA2219 aluminiumecopper alloy was *** backscattered diffraction results established that the grain size and orientation of weld nugget of triangle profile is finer than that of conical *** scanning calorimetric results show the evidence of precipitate dissolution during *** was found that the microstructure changes,such as grain size and its orientation precipitate dissolution during FSW influence the hardness and corrosion *** corrosion resistance of friction stir welds of AA2219 was found to be better for triangle profile tool compared to conical profile which is attributed to material flow and strengthening precipitate morphology in various *** amount of heat generation during FSW made using triangle profile tool may be the reason for greater dissolution of strengthening precipitates in nugget zone and coarsening in thermo mechanically affected zone(TMAZ) and heat affected zone(HAZ).

关键词： Al-Cu合金搅拌摩擦焊接微观结构轮廓点蚀摩擦搅拌焊接电子背散射衍射机械性能

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Microstructure and pitting corrosion of shielded metal arc welded high nitrogen stainless steel

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Defence Technology（防务技术） 2015年第3期11卷 237-243页

作者： RAFFI MOHAMMED G.MADHUSUDHAN REDDY k.srinivasa rao Department of Metallurgical Engineering Andhra University Defence Metallurgical Research Laboratory

The present work is aimed at studying the microstructure and pitting corrosion behaviour of shielded metal arc welded high nitrogen steel made of Cromang-N electrode. Basis for selecting this electrode is to increase the solubility of nitrogen in weld metal due to high chromium and manganese content. Microscopic studies were carried out using optical microscopy(OM) and field emission scanning electron microscopy(FESEM). Energy back scattered diffraction(EBSD) method was used to determine the phase analysis, grain size and orientation image mapping. Potentio-dynamic polarization testing was carried out to study the pitting corrosion resistance in aerated 3.5% NaCl environment using a GillAC electrochemical system. The investigation results showed that the selected Cr-Mn-N type electrode resulted in a maximum reduction in delta-ferrite and improvement in pitting corrosion resistance of the weld zone was attributed to the coarse austenite grains owing to the reduction in active sites of the austenite/delta ferrite interface and the decrease in galvanic interaction between austenite and delta-ferrite.

关键词：耐点蚀性能高氮不锈钢焊缝金属电弧焊接研究组织屏蔽场发射扫描电子显微镜奥氏体晶粒

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Influence of tool pin profile on microstructure and corrosion behaviour of AA2219 Al-Cu alloy friction stir weld nuggets

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Defence Technology（防务技术） 2015年第3期11卷 197-208页

作者： Ch.VENkATA rao G.MADHUSUDHAN REDDY k.srinivasa rao Department of Metallurgical Engineering Andhra University Defence Metallurgical Research Laboratory

To overcome the problems of fusion welding of aluminium alloys, the friction stir welding(FSW) is recognized as an alternative joining method to improve the mechanical and corrosion properties. Tool profile is one of the important variables which affect the performance of the FS weld. In the present work, the effect of tool profile on the weld nugget microstructure and pitting corrosion of AA2219 aluminium-copper alloy was studied. FSW of AA2219 alloy was carried out using five profiles, namely conical, square, triangle, pentagon and hexagon. The temperature measurements were made in the region adjacent to the rotating pin. It was observed that the peak temperature is more in hexagonal tool pin compared to the welds produced with other tool pin profiles. It is observed that the extensive deformation experienced at the nugget zone and the evolved microstructure strongly influences the hardness and corrosion properties of the joint during FSW. It was found that the microstructure changes like grain size, misorientation and precipitate dissolution during FSW influence the hardness and corrosion behaviour. Pitting corrosion resistance of friction stir welds of AA2219 was found to be better for hexagon profile tool compared to other profiles, which was attributed to material flow and strengthening precipitate morphology in nugget zone. Higher amount of heat generation in FS welds made with hexagonal profile tool may be the reason for greater dissolution of strengthening precipitates in nugget zone.

关键词： Al-Cu合金搅拌摩擦焊微观结构焊缝区刀具腐蚀行为焊接接头性能耐腐蚀性能

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Microstructure,mechanical and corrosion behavior of high strength AA7075aluminium alloy friction stir welds-Effect of post weld heat treatment

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Defence Technology（防务技术） 2015年第4期11卷 362-369页

作者： P.Vijaya kumar G.Madhusudhan Reddy k.srinivasa rao Department of Mechanical Engineering R.I.T. Defence Metallurgical Research Laboratory Department of Metallurgical Engineering Andhra University

High strength aluminium alloy AA7075(Al-Zn-Mg-Cu) is a precipitate hardenable alloy widely used in the aerospace,defense,marine and automobile *** of the heat treatable aluminium alloys in all these sectors is ever-increasing owing to their excellent strength-toweight ratio and reasonably good corrosion *** shortage in corrosion resistance,however,usually poses negative concern about their reliability and lifetime when they service in the variable marine *** alloys also exhibit low weldability due to poor solidification microstructure,porosity in fusion zone and lose their mechanical properties when they are welded by fusion welding *** stir welding(FSW) is a reliable technique to retain the properties of the alloy as the joining takes place in the solid *** welds are susceptible to corrosion due to the microstructural changes in the weld nugget during *** this work,the effect of post weld treatments,viz.,peak aging(T6) and retrogression & reaging(RRA),on the microstructure,mechanical properties and pitting corrosion has been *** stir welding of 8 mm-thick AA7075 alloy was carried *** microstructural changes of base metal and nugget zone of friction stir welds were studied using optical microscopy,scanning electron microscopy and transmission electron *** and hardness test of base metal and welds has been carried *** corrosion resistance was determined through dynamic polarization *** was observed that the hardness and strength of weld were observed to be comparatively high in peak aged(T6) condition but the welds showed poor corrosion *** resistance to pitting corrosion was improved and the mechanical properties were maintained by RRA *** resistance to pitting corrosion was improved in RRA condition with the minimum loss of weld strength.

关键词： 7075铝合金高强度铝合金搅拌摩擦焊耐腐蚀性热处理效果微观结构焊缝机械性能

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Friction stir surfacing of cast A356 aluminiumesilicon alloy with boron carbide and molybdenum disulphide powders

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Defence Technology（防务技术） 2015年第2期11卷 140-146页

作者： R.SRINIVASU A.SAMBASIVA rao G.MADHUSUDHAN REDDY k.srinivasa rao Department of Metallurgical Engineering Andhra University Defence Metallurgical Research Laboratory

Good castability and high strength properties of Ale Si alloys are useful in defence applications like torpedoes,manufacture of Missile bodies,and parts of automobile such as engine cylinders and *** wear resistance of the alloys is major limitation for their *** stir processing(FSP) is a recognized surfacing technique as it overcomes the problems of fusion route surface modification *** in view of the requirement of improving wear resistance of cast aluminiumesilicon alloy,friction stir processing was attempted for surface modification with boron carbide(B4C) and molybdenum disulfide(Mo S2) ***,micro compositional analysis,hardness and pin-on-disc wear testing were used for characterizing the surface composite *** study revealed breaking of coarse silicon needles and uniformly distributed carbides in the A356 alloy matrix after *** and uniformity in hardness was obtained in surface composite *** wear resistance was achieved in friction stir processed coating with carbide *** of solid lubricant Mo S2 powder was found to improve wear resistance of the base metal significantly.

关键词： A356合金二硫化钼堆焊技术硅合金钼粉末碳化硼摩擦铸铝

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Microstructure and pitting corrosion of armor grade AA7075 aluminum alloy friction stir weld nugget zone-Effect of post weld heat treatment and addition of boron carbide

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Defence Technology（防务技术） 2015年第2期11卷 166-173页

作者： P.VIJAYA kUMAR G.MADHUSUDHAN REDDY k.srinivasa rao Department of Mechanical Engineering R.I.T. Defence Metallurgical Research Laboratory Department of Metallurgical Engineering Andhra University

Friction stir welding(FSW) of high strength aluminum alloys has been emerged as an alternative joining technique to avoid the problems during fusion *** recent times FSW is being used for armor grade AA7075 aluminum alloy in defense,aerospace and marine applications where it has to serve in non uniform loading and corrosive *** though friction stir welds of AA7075 alloy possess better mechanical properties but suffer from poor corrosion *** present work involves use of retrogression and reaging(RRA) post weld heat treatment to improve the corrosion resistance of welded joints of aluminum *** attempt also has been made to change the chemical composition of the weld nugget by adding B4C nano particles with the aid of the FSW on a specially prepared base metal plate in butt *** effects of peak aged condition(T6),RRA and addition of B4C nano particles on microstructure,hardness and pitting corrosion of nugget zone of the friction stir welds of AA7075 alloy have been *** though RRA improved the pitting corrosion resistance,its hardness was slightly *** improvement in pitting corrosion resistance was achieved with addition of boron carbide powder and post weld heat treatment of RRA.

关键词： 7075铝合金搅拌摩擦焊焊后热处理碳化硼颗粒抗点蚀微观结构装甲 7075合金

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Enhancement of wear and ballistic resistance of armour grade AA7075 aluminium alloy using friction stir processing

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Defence Technology（防务技术） 2015年第1期11卷 10-17页

作者： I.SUDHAkAR V.MADHU G.MADHUSUDHAN REDDY k.srinivasa rao Department of Mechanical Engineering MVGRCE Vizianagaram India Defence Metallurgical Research Laboratory Hyderabad India Department of Metallurgical Engineering Andhra University Visakhapatnam India

Industrial applications of aluminium and its alloys are restricted because of their poor tribological properties. Thermal spraying, laser surfacing, electron beam welding are the most widely used techniques to alter the surface morphology of base metal. Preliminary studies reveal that the coating and layering of aluminium alloys with ceramic particles enhance the ballistic resistance. Furthermore, among aluminium alloys,7075 aluminium alloy exhibits high strength which can be compared to that of steels and has profound applications in the designing of lightweight fortification structures and integrated protection systems. Having limitations such as poor bond integrity, formation of detrimental phases and interfacial reaction between reinforcement and substrate using fusion route to deposit hard particles paves the way to adopt friction stir processing for fabricating surface composites using different sizes of boron carbide particles as reinforcement on armour grade 7075 aluminium alloy as matrix in the present investigation. Wear and ballistic tests were carried out to assess the performance of friction stir processed AA7075 alloy. Significant improvement in wear resistance of friction stir processed surface composites is attributed to the change in wear mechanism from abrasion to adhesion. It has also been observed that the surface metal matrix composites have shown better ballistic resistance compared to the substrate AA7075 alloy. Addition of solid lubricant Mo S2 has reduced the depth of penetration of the projectile to half that of base metal AA7075 alloy. For the first time, the friction stir processing technique was successfully used to improve the wear and ballistic resistances of armour grade high strength AA7075 alloy.

关键词： Armour grade aluminium alloy Friction stir processing Boron carbide Molybdenum disulphide Wear Ballistic resistance

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