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Interrupted Boriding of Medium-carbon Steels

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  • Interrupted Boriding of Medium-Carbon Steels

    P. GOPALAKRISHNAN, P. SHANKAR, M. PALANIAPPA, and S.S. RAMAKRISHNAN

    The results of an extensive study on the microstructure, microhardness, corrosion, and tensile propertiesof continuously borided and interrupted borided specimens of medium-carbon steel are compared.Carbon repartitioning away from the surface is one of the principal modes to accommodate the highstrains introduced on boron diffusion into the case. However, this is a kinetically constrained processand is more predominant on interrupted boriding. The effect of such a carbon redistribution is toresult in microstructural modifications including (1) blunting of boride needle tips, (2) precipitationof nearly spherical and fine borocarbides, and (3) enhanced carbon segregation at the boride needle/steel matrix interface on interrupted boriding. The mechanisms aiding the change in the morphologyof the boride needles are discussed. The improvements in the mechanical and corrosion propertiesof the interrupted borided specimens over continuously borided specimens are described.

    I. INTRODUCTION of boron carbide and borax mixtures.[5,6] Alternatively, boroncarbide or amorphous boron has been used as the boronTHE surfaces of engineering components are subjectedsource, along with diluents such as silicon carbide or aluminato higher stresses and greater fatigue, abrasion, and corrosiveor graphite and activators like KBF4 or NaF in the boridingdamages than the interior. Therefore, more than 90 pct ofmixtures.[4,6,810] Ferroboron can be considered as a boronthe service failures of engineering components initiate at,source instead of boron carbide. However, it is reported that

    or near, the surface. Surface modification techniques arethe commercial grades of ferroboron contain impurities like

    employed to improve the resistance to failure by producing Si and Al; therefore, the use of this material leads to aa hard and wear-resistant case around a soft and tough core. degenerate layer. However, special-quality ferroboron canTwo major classes of treatments available for enhancing the be used to get a good-quality boride case, according to thesurface properties are thermal and thermochemical. Thermal

    classical review book by Von Matuschka,[11] which is atreatments, such as flame and induction hardening, modifycomprehensive, general, and theoretical compendium onthe microstructure without modifying the surface chemistry, boriding.

    whereas in thermochemical methods, the surface chemistry The molten salt boriding process is usually performedis altered. Carburizing and nitriding are well known thermo-using anhydrous borax, mixed with reducing agents like

    chemical methods.[1] Boriding or boronizing is a recent boron carbide, silicon carbide, or silico-calcium.[12] Amor-process, which is analogous to carburizing and nitriding. phous boron is stated to be better than boron carbide, sinceBoriding can develop surface hardness in the range of it produces less slurry.[13] Electrolytic boriding is carried out1500 to 2000 HV, as compared to a hardness in the range in inert atmospheres like argon, using borax-based melts atof 600 to 1100 HV for nitriding, 700 to 850 HV for carburiz-

    about 1173 K at a current density of about 0.15 to 0.25 A/ing, and 950 to 1100 HV for chromium plating. Boridedcm2.[14,15,16] Electrolytic boriding produces thicker coatingslayers provide a wear resistance comparable to that of sint- in relatively shorter times compared to molten salt or pack

    ered carbides. The wear resistance of cold-working tools is boriding processes.[16] There are many other techniques forincreased by about 10 times and that of hot-working tools boriding, e.g., gas boriding, vacuum boriding, plasma borid-and dies by about 3 times as a result of boriding.[2] If the ing, and thermal spraying of powders.[2,12,17] Out of all theseprocess has been performed properly and the right material

    techniques, pack and molten salt (electroless) processes areand layer thickness have been chosen, boriding can extend

    technologically simpler and more economical compared tothe service life of engineering components beyond thatother boriding processes.imparted by traditional methods like carburizing or During boriding of plain-carbon steels, needle-like FeB

    nitriding.[3,4]and Fe2B phases are formed. When the boron potential isVarious processes adopted for boriding include pack bor- low, Fe2B phase alone forms in the case. At higher potentialsiding, molten salt boriding, electrolytic boriding, gas borid-of boron, FeB phase also forms along with Fe2B. The Fe2Bing, vacuum boriding, etc. In pack boriding, the sample is phase forms adjacent to the core and the FeB phase forms

    cleaned and kept surrounded by solid mixtures consistingnear the surface.[4,10,12,18] It is reported that FeB19 phase alsoforms,[19] very rarely, when the boron potential is very high.A single-phase structure is desirable.[6] Fe2B is preferred to

    P. GOPALAKRISHNAN, Assistant Professor, Department of Metallurgi- FeB,[10,20,21] since FeB is very hard and brittle and has acal Engineering, and S.S. RAMAKRISHNAN, Dean, School of Metallurgy

    coefficient of thermal expansion that differs from the matrixand Materials, and Professor, Department of Metallurgical Engineering,by a factor of 3 and that causes spalling during cooling.[22]are with PSG College of Technology, Coimbatore 641 004, India. P.

    SHANKAR, Scientist, is with the Metallurgy and Materials Group, Indira Although boriding of plain-carbon steels improves theGandhi Centre for Atomic Research, Kalpakkam 603 102, India. M. wear, abrasion, fatigue, tensile, corrosion, corrosion-fatigue,PALANIAPPA, formerly ME Student, Department of Metallurgical Engi-

    and oxidation properties,[1,5,6,8,13,14,23,24] one serious draw-neering, PSG College of Technology, is Doctoral Scholar, Indian Institute back is the brittleness of the case.[6,23] Several methods haveof Technology, Madras 600036, India.

    Manuscript submitted September 25, 2000. been attempted to solve this problem. They include (1) partial

    METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 33A, MAY 20021475

  • Fig. 2Interrupted boriding cycle.

    Fig. 1Pack boriding setup.

    deboriding of the saturated layer,[6] (2) the addition of copperto the borided layer,[6] (3) multicomponent diffusion impreg-

    Fig. 3Tension test sample.nation of iron with elements like silicon, aluminium, etc.,along with boron,[25] (4) superplastic boronizing, which pro-duces a nonacicular structure with eqiaxed grains of theborides, thereby reducing brittleness,[26] and (5) laser surface B. Thermal Treatmentmodification of borided surfaces.[27,28,29]

    1. Continuous processThe objectives of the present study are (1) to developThe assembly shown in Figure 1 was loaded in the furnacelow-cost boriding processes using low-cost and easily avail-

    at around 1073 K, and it was further heated to the boridingable raw materials, (2) to optimize the process parameterstemperature (1223 K). When the temperature reached 1223to get only Fe2B phase, resulting in better toughness, and K, boriding was performed continuously for 4 hours. After(3) to improve the microstructural morphology and, thereby,4 hours, the crucible was removed from the furnace andincrease the toughness and ductility of the boride layer byallowed to cool in still air.an interrupted thermocyclic boriding process.

    In the case of the molten salt process, the sample to beborided was kept in the boriding melt and boriding was

    II. EXPERIMENTAL PROCEDURE carried out at 1223 K for 4 hours continuously. After 4hours, the sample was removed from the melt and allowedIn this study, boriding was performed using pack processesto cool in air.and molten salt (electroless) processes. The medium-carbon

    steel (0.45 pct C, 0.2 pct Si, 0.7 pct Mn, 0.05 pct S, and 2. Interrupted process0.05 pct P) specimens were ground with a rough emery In interrupted boriding process, after every hour of borid-paper to remove surface oxides. They were subsequently ing, the crucible (with the stainless steel box containing thecleaned in a 50 pct aqueous solution of HCl and dried. boriding mix and sample) was removed from the furnace

    and allowed to cool in still air. During cooling, a thermocou-ple was inserted in the sand so as to touch the stainless steelA. Boriding Methodsbox. When the temperature was about 873 K, the crucible1. Pack boriding processwas again loaded into the furnace at 1223 K. Boriding wasInitially, boron carbidebased powders were employed to done for 1 hour at 1223 K, and the sample was cooled againdevelop the boride layer. Subsequently, ferroboron-based to about 873 K. This procedure was repeated four times.powders were also used to get a boride layer. Fine powders The thermal cycle is shown in Figure 2. After the interrupted

    of the mixture were taken in a stainless steel container. The boriding process (four steps of 1 hour each), the cruciblemedium-carbon steel [EN8] sample of 10 mm in diameter

    was removed from the furnace and kept at 873 K for 1and 10 mm in length was placed inside the box in such hour in a separate furnace. It was then cooled in air toa way that the sample was uniformly surrounded by an

    room temperature.approximately 15-mm-thick boriding mixture on all sides. In the case of molten salt boriding, the sample wasThe box was tightly closed, and the gap between the lid and

    removed from the melt after every hour of boriding, cooledthe box was sealed with fireclay. The box was placed in a for 5 minutes in air, and again placed in the melt. The totalcrucible (clay-graphite) surrounded by silica sand, as shown boriding time was 4 hours. After this boriding treatment,in Figure 1. the sample was heated for 1 hour at 873 K and then cooled

    in air.2. Molten salt (el