UDK 621.785:669.14.018:620.179.11 ISSN 1580-2949 Izvirni znanstveni članek MATER. TEHNOL. 34(6)425(2000) B. MATIJEVI], M. STUPNIŠEK: THE DIFFUSION GROWTH OF CARBIDE LAYERS ON STEEL SURFACES THE DIFFUSION GROWTH OF CARBIDE LAYERS ON STEEL SURFACES DIFUZIJSKA RAST KARBIDNE PLASTI NA POVRŠINI JEKLA Božidar Matijevi}, Mladen Stupnišek Faculty of Mechanical Engineering and Naval Architecture, University of Zagreb, I. Luči}a 5, 10000 Zagreb, Croatia bozidar.matijevicŽfsb.hr Prejem rokopisa - received: 2000-10-10; sprejem za objavo - accepted for publication: 2000-11-10 This paper discusses the diffusionprocess of carbide-layer formationonsteels. Applying the principles of physical chemistry, a mathematical model of the process has been set up and a computer program for calculating technological parameters has been developed. Based on the results of the experimental work and literature data, the assumption that the speed of the process is proportional to the thermodynamic activity of carbon in the austenite has been proven. Key words: carbide layers, vanadium carbide, diffusion V delu je opisan difuzijski proces rasti karbidne plasti na površini jekla. Upoštevajoč principe fizikalne kemije smo naredili matematični model procesa in računalniški program za izračun tehnoloških parametrov procesa. Na podlagi rezultatov eksperimentalnega dela in literaturnih virov smo potrdili, da je hitrost procesa premo sorazmerna aktivnosti ogljika v avstenitu. Ključne besede: karbidne plasti, vanadijev karbid, difuzija 1 INTRODUCTION The process of forming carbide layers on steels by diffusion is of growing importance for industrial applications because of its advantages over other processes. The procedure is usually performed ina salt bath of special composition which contains the carbide-forming element vanadium, but might also contain some other elements, e.g. Cr, Nb1,2. At elevated temperatures from 900 to 1000 °C a moltenbath of borax containing vanadium, either in the form of metal or an oxide, results in atomic vanadium forming on the surface of the substrate, which is usually steel. Entering the austenite lattice, the vanadium diffuses from the surface into the substrate, up to the solubility limit which depends on temperature and chemical composition of steel matrix. At this point the reaction of vanadium and carbonresults inthe formationof vanadium carbides. Through the formed carbide layer the carbonmoves towards the newly generated surface where the process of forming carbides continues, thus increasing the thickness of the carbide layer. Carbon moves from the inner layers of the substrate towards the surface by diffusion. As carbon from the austenite is used in forming the vanadium carbide, its concentration beneath the carbide layer is substantially lower than that of the substrate3,4. As a consequence, carbon is the most important element for the formation of carbides. The carbide layer has a high hardness and a high wear resistance to abrasion, adhesion and tribocorrosion, which is particularly important for machine parts and tools. The thickness of the layer is in the range 3 to 20 µm. With the aim of better understanding the process and improving applications, research into the metallurgical fundamentals of the process was carried out. 2 A MODEL FOR CARBIDE-LAYER FORMATION By dipping the part of steel into the molten bath the carbide layer is formed onthe steel surface. The diffusionprocesses that take place during the formation of the carbide layer onthe surface are very complex. The formationof the vanadium-carbide layer VC0.88 onthe steel surface is explained using the example of vanadizing, where vanadium is added to a molten bath as a metal or as anoxide. At the steel surface vanadium is combines with carbondissolved inthe austenite at the processing temperature. Through the formed carbide layer, the carbondiffuses to the newly formed surface, where it is combined with vanadium, thus increasing the layer thickness. At the same time, vanadium diffuses to the surface layer of the steel through the carbide layer. The rate of vanadium diffusionis lower thanthat of carbonthrough the carbide layer. As a result, the thickening of the carbide layer is not from the surface towards the center, like inthe other diffusionprocesses (carburizing, nitriding, boriding, …), but happens due to the growth of the carbide layer onthe steel surface. Figure 1 shows anSEM image of the vanadium-carbide layer with the marked analysing line (L2) and the AES spectrum of iron, carbon and vanadium in, and below, the vanadium-carbide layer. The kinetics of the process of carbide-layer formation and the thickness of the formed layer depend on the temperature and the duration of the process and on the MATERIALI IN TEHNOLOGIJE 34 (2000) 6 425 B. MATIJEVI], M. STUPNIŠEK: THE DIFFUSION GROWTH OF CARBIDE LAYERS ON STEEL SURFACES Figure 1: (a) SEM image of vanadium-carbide layer; (b) AES spectrum Fe, C and V measured along the line L2. IMT Report 22/99 Slika 1: (a) SEM-slika plasti vanadijevega karbida, (b) AES-spekter Fe, C inV na črti L2. Poročilo IMT 22/99 content of carbon and alloying elements in the steel. Previous research suggests that the carbide layer thickness is linearly proportional to the carbon concentration in the steel5. However, inorder to analyse the influence of the chemical composition of the steel on the process it is necessary to consider the quantity of carbon in the austenite at the processing temperature, not the total content of carbon in the steel. This is of particular importance for high-carbon high-alloyed steels, which have one part of the carbon combined in the carbides at the processing temperature. These carbides insteel are mostly stable carbides of chromium, tungsten and other carbide-forming elements. It is necessary to determine how mach of the total carbon is insolid solutioninthe austenite, because only atomic carbonparticipates inthe process. Isothermic diagrams provide a quantitative presentation and enable identifi-cationof the carbondistributionand the distributionof other elements in the austenite and carbide. According to the principles of physical chemistry, the gradient of the chemical potential of the diffusing elements is the driving force of the diffusion processes. The concept, which states that thermodynamic activity of carbon in austenite controls the process of diffusion of carbon, has beenconfirmed inthe carburizing process6. At constant temperature, instead of the chemical potentials, thermodynamic activities of the diffusing elements may be used. Numerous authors have investigated both the two-component Fe-C system and multi-component systems for the thermodynamic activity of carbon within austenite. The Thermodynamic activity of carbon depends onthe chemical compositionof the steel and the processing temperature. There are empirical equations for the quantitative influences7,8,9. log aC 2300 2.21 + 0.15 -%C + log %C (1) logaa C'oy logaCF e-C 0.055-%Si - 0.013-%Mn - 0.04-%Cr + 0.014-%Ni -- 0.013-%Mo - 0.102-% V- 0.014-%Al - 0.006-%Cu + 0.22-%N (2) The thickness of the diffusion layer for most diffusionprocesses increases inproportionto the square root of the processing time and can be calculated using the equation: d 2 = k-t (3) where d is the thickness of the layer in m; k is the layer growth-rate constant in m2/s and t is processing time in s. The layer growth-rate constant depends on the processing temperature, since for heat-activated processes the exponential relationship of growth rate according to the Arrhenius equation is valid: k = k0 -e Q RT (4) where k0 is the frequency factor in m2/s; Q is the activationenergy of the process inJ/mol; R is the gas constant in J/ mol K and T is the absolute temperature measured inK. Taking the logarithm of the Arrhenius equation the following expression is obtained: ln k = ln k (5) It provides the possibility to graphically determine the activationenergy of the process for the formationof vanadium carbide (Q) and the frequency factor (k0) using a "ln k - 1/T" diagram. The intersection of "ln k" in the "ln k - 1/T" diagram determines the value of the frequency factor (ln k0) and the slope of the Q/R line determines the activation energy of the process (Q). T 426 MATERIALI IN TEHNOLOGIJE 34 (2000) 6 B. MATIJEVI], M. STUPNIŠEK: THE DIFFUSION GROWTH OF CARBIDE LAYERS ON STEEL SURFACES 3 EXPERIMENTAL DETAILS Since data relating to values of the activation energy of the process (Q) and the frequency factor (k0) for the vanadizing process are not available in the literature, three different carbon steels of grades DIN: Ck 45, Ck 60 and C 105 W1 have been investigated. The chemical compositionof the steels was determined by bulk spectroscopic analysis using glow discharge. The results are showninTable 1. Table 1: Chemical analysis of the tested steels Tabela 1: Kemična analiza preizkušenih jekel Steels grade C % Si % Mn % S % P % Ck45 0.49 0.33 0.69 0.027 0.013 Ck60 0.68 0.27 0.87 0.025 0.014 C 105 W1 0.98 0.15 0.25 0.024 0.013 The temperature and duration of the vanadizing process were determined according to the model for a central composite experiment, which has certain advantages in planning experimental work for heat-activated processes10. In accordance with the experimental plan, the following temperatures were selected: 930, 950, 1000, 1050 and 1070 °C, and times: 60, 110, 240, 370 and 420 minutes. Vanadizing is performed in a salt bath for forming the carbide layer, which was developed in the course of previous studies4. After vanadizing process, the samples were cut ina directionperpendicular to the carbide layer and prepared for micrographic testing. The depth of the vanadium-carbide layer on the metallo-graphically prepared samples was measured with a LECO 2001 image analyser. From the measured thicknesses of the vanadium carbide, values for the carbide-layer growth-rate constant (k) at the temperatures 930, 950, 1000, 1050 and 1070 1/T, 1/ K 7.4E-4 7.6E-4 7.8E-4 8.0E-4 8.2E-4 8.4E-4 -30.00 -31.00 c -32.00 , , * - «K - .. %**, *Č- C*6o č" "" - + č+ Č Č