The benefits of using 3 gas mixture low pressure carburizing (LPC) for high alloy steels R. Gorockiewicz, Ph.D University of Zielona Góra, Poland A. Adamek, M. Korecki SECO/WARWICK S.A., Swiebodzin, Poland Abstract The optimum utility features of the parts under treatment depend on both the carbon profile in the layer as well as on the Low pressure carburizing (LPC) in vacuum furnaces has many microstructure, which is a function of carbon profile, steel applications in industrial technologies where high quality, alloy additions, conditions of cooling while hardening and reliability and repeatability are required. High quality work is parameters of deep-freeze (cryogenic) treatment and achieved using the patented process control method, tempering. The characteristic feature of the microstructure is FineCarb®, that delivers a precise three gas carburizing the fact that in the hardened layer, apart from martensite, there mixture injection sequence controlled through a computerized appear carbides, the kind, number, size and morphology of supervision system and process simulation software – which affect utility features of the parts. Steel vacuum SimVac™. carburizing is not an equilibrium process, thus in order to determine the process parameters expert systems are The advantage of the LPC method is demonstrated in high employed, such as FineCarb [1] technology, which, based on alloy and special steels, where a high temperature carburizing mathematical models and computer simulations as well as on process is used and rapid carbon diffusion reduces the process the grounds of experimental verification can run the process time significantly. In addition, extended hardenability of these with a very high accuracy and repeatability. steels enables effective gas quenching that reduces distortion as well as manufacturing costs. Vacuum furnaces with gas This paper presents some of the results of vacuum carburizing quenching are a green manufacturing technology, a major by this method along with high pressure gas quenching of improvement over traditional atmosphere technologies. selected special and high-alloy steel obtained in actual trials. The article describes actual process trials, results and Special and high-alloy steel grades conclusions carried out with high alloy and special steel grades: Ferrium C61, CSB-50NIL, 6-2-5 and X5CrNiMo17- The main alloy components of special steel is chromium Cr 12-2, which have many applications in both automotive and (11-18% of the mass) and molybdenum Mo (0,5-4.5%) and aerospace. also, depending on the steel grade: nickel Ni (0,2 -4% of the mass, but also 6-13%), vanadium V (0.1-1.5%) and in some Introduction cobalt Co (10-18% of the mass). In case of high-alloy steel grades the main alloy component is molybdenum Mo (3-5% of Currently, it is common to search for and apply new materials, the mass) or alternately wolfram W, chromium Cr (0,2-4.5%), especially in automotive and aerospace, in order to limit the nickel Ni (0,2-3.5%) vanadium V (0,1-1,5%). weight and improve the performance and durability of the end product. The features of these materials, characterized by an Table 1 presents several examples of advanced types of steel. unconventional chemical composition, are ideally suited to The majority of them have low carbon content and the precision heat treatment carried out in vacuum furnaces. Gear required utility features are obtained through carburizing and wheels, pinions, drive system bearings, pump bearings, hardening or heat treatment at all. camshafts and other parts in flight control equipment are most often made of high-alloy and special steel that underwent vacuum carburizing. Final qualities are ensured by hardening often connected with deep-freeze (cryogenic) treatment and tempering. Due to high, changing loads and/or operation in high temperatures, parts that are subject to treatment require layers that are 0.25-6.5 mm thick hardened case with a strength and ductile core. The time necessary for making such thick layers can be reduced by high temperature carburizing. Oil quenching and more frequent hardening in chambers provided with high pressure gas cooling ensures conversion of austenite into martensite in the carburized layer and the core. Table 1. Examples of advanced special steel types used on flow controllers (MFC) ensuring repeatability of the settings high-load parts [2]. and flows. The line of MFCs is provided with built-in appropriate pressure sensors and cut-off valves – manual and electromagnetic. The supply of the carburizing mixture in the diffusion cycles is held, whereas the pressure in the chamber is lowered by the pumping system to the level of 0.05 mbar or controlled at the level of partial pressure by means of nitrogen. At the same time, as a result of carbon diffusion out of the edge zone inside the material, the carbon concentration on the charge surface is lowered and the thickness on the carburized layer increases. The example of the parameters and their course in the carburizing process is illustrated in figure 2. Vacuum carburizing LPC The vacuum furnaces (figure 1) were equipped with a low pressure, FineCarb, carburizing system. This system ensures thermochemical process within the temperature range of 800- 1100oC (1470-2010oF) and within the pressure range of 0.5-10 mbar [3]. Figure 2. FineCarb low pressure carburizing process cycle The carbon profile in the layer being carburized on a particular steel type and in a chosen temperature range depends on a number and the length of carburizing and diffusion cycles. The process ensures that the carburized layer obtained is uniform even on the oddly shaped details; this process provides the capability of carburizing deep holes. The process is clean (no Figure 1. Vacuum furnace, LPC + HPGQ creation of hydrocarbons in a form of soot, resin or tar). Type 15.0 VPT-4050/48N The carburizing system goes with a computer software, This process of low pressure carburizing is based on SimVac, which enables design of carburizing parameters of alternating and cyclic application of carburizing - boost and the typical steel used after carburizing, so that the details can diffusion phase – soaking on the temperature [4]. During get the required carburized layer, most frequently defined by carburizing cycles the carburizing atmosphere in a given its thickness and surface carbon concentration, figure 3. The amount and time at the pressure range of 0.5 – 10 mbar is control and supervision system is fully automated which leads provided to the process chamber. The carburizing atmosphere to repeatability of the processes and to improvement of quality is a mixture of three gases (acetylene, ethylene and hydrogen), of heat treatment. the composition of which was developed and patented by SECO/WARWICK and the Technical University of Łódź [5]. Chemical reactions taking place in the atmosphere and the catalytic effect of the charge surface lead to the formation of active carbon atoms, which are absorbed by austenite, saturating it and then diffusing inside the material. Dosing of the carburizing atmosphere is performed through precise mass were carried out in a single chamber vacuum furnace – figure 1, which allows carburizing, hardening by means of a high pressure gas quench and tempering. The results of the thermochemical treatment were evaluated by measuring profile of case hardness and carbon concentration, and changes of microstructure in the cross-section of the tested samples Ø 25x10 mm (1.0x0.4”) and 25x 150 mm (1.0x6.0”). Transverse microsections of the samples Ø 25x10mm (1.0x0.4”) were used for measuring changes of hardness HV0.1 or HV0.5 and microstructures, and rollers of Ø 25x150mm (1.0x6.0”) were used for evaluating carbon concentration at the depth of the carburized layer. Hardness profile was measured with a microhardness tester FM-700 (Future-Tech) and surface hardness was measured with Wilson Wolpert Testor 751. The microstructure was observed by means of an optical microscope Neophot and the scanning Figure 3 SimVac Simulation Software microscope Jeol 5600 LV, while metallographic specimen were digested with Nital, Adler’s reagent. Carbon concentrations at particular depths were measured with Leco Hardening in HPGQ vacuum furnaces tester by burning chips 0.05mm (0.002”) thick, which were taken by turning rollers Ø 25x150 mm (1.0x6.0”) previously Special and high-alloy steels have a high hardenability due to tempered at the temperature of 650oC (1200oF). a large content of alloy additives, so that they can be hardened efficiently in nitrogen under 15 bar pressure. Single-chamber furnaces already provide sufficient cooling speed (300-600 Material Ferrium C61 [8] W/m2K) to obtain the right structure of the carburized layer and the core of the parts under treatment [6]. Gas quenching An advanced steel grade with improved utility features is the has many advantages in comparison with the traditional oil alloy Ferrium C61 (Questec Innovations). hardening: This alloy belongs to a new group of martensite steel grades • Environmentally friendly used for bearings and power transmissions. Ferrium C61 is an • Labor safety, excellent alternative to typical materials used in gears, where • Decreased investment and operational expenses via the parts are required to have improved mechanical properties, shorter manufacturing cycle due to the elimination of but are not possible to be redesigned. Ferrium C61 was the washing process after oil quenching especially elaborated to ensure a high hardness of carburized surfaces (60-62 HRC) and possesses good tribological • Reduction of post heat treatment activities. properties, ductility and fatigue resistance similar to the properties of typical steel grades used in such applications as An important benefit is the potential to minimize deformation AISI 9310 and EN36C, and apart from that it is also possessed through an optimum selection of the cooling speed and with additional very high core strength. direction as a result of controlling the pressure of the gas and its velocity and the area of inflow.