→ Expert edition Furnace atmospheres no. 1. Gas carburising and carbonitriding. 02 Gas carburising and carbonitriding Preface. This expert edition is part of a series on process application technology and know-how available from Linde Gas. It describes findings in development and research as well as extensive process knowledge gained through numerous customer installations around the world. The focus is on the use and control of furnace atmospheres; however a brief introduction is also provided for each process. 1. Gas carburising and carbonitriding 2. Neutral hardening and annealing 3. Gas nitriding and nitrocarburising 4. Brazing of metals 5. Low pressure carburising and high pressure gas quenching 6. Sintering of steels Gas carburising and carbonitriding 03 Passion for innovation. Linde Gas Research Centre Unterschleissheim, Germany. With R&D centres in Europe, North America and China, Linde Gas is More Information? Contact Us! leading the way in the development of state-of-the-art application Linde AG, Gases Division, technologies. In these R&D centres, Linde's much valued experts Carl-von-Linde-Strasse 25, 85716 Unterschleissheim, Germany are working closely together with great access to a broad spectrum of technology platforms in order to provide the next generation of [email protected], www.heattreatment.linde.com, atmosphere supply and control functionality for furnaces in heat www.linde.com, www.linde-gas.com treatment processes. As Linde is a trusted partner to many companies in the heat treatment industry, our research and development goals and activities are inspired by market and customer insights and industry trends and challenges. The expert editions on various heat treatment processes reflect the latest developments. 04 Gas carburising and carbonitriding Contents. Preface. 2 4. Atmosphere generation, gas supply 18 4.1 Carburising atmospheres 18 Passion for innovation 3 4.1.2 Endogas 20 4.1.3 Other carburising atmospheres 21 4.2 Gas and methanol supply 21 1. Introduction 6 4.2.1 Nitrogen supply 21 4.2.2 Methanol supply 23 4.2.3 Supply of other gases 23 2. Process 7 4.2.4 Flow and composition control 23 2.1 Basic principles 7 2.2 Steel properties 9 3. Furnace and equipment 14 3.1 Furnaces 14 3.2 Atmosphere equipment 16 3.2.1 High-speed gas injection for improved gas homogeneity 16 3.2.2 Methanol injectors and pumps 17 3.2.3 Sample gas handling 17 Gas carburising and carbonitriding 05 5. Atmosphere control 26 6. Safety 38 5.1 Theory and principles 26 6.1 Safety awareness 38 5.1.1 Carbon mass transfer and chemical equilibrium 26 6.2 Gases used in the process 38 5.1.2 Nitrogen mass transfer and chemical equilibrium 28 6.3 Potential safety hazards and their sources 38 5.1.3 Carbon potential control 29 6.3.1 Explosions/flammability/fire 39 5.1.4 Single step or boost carburising 31 6.3.2 Toxicity and asphyxiation 40 5.1.5 Carbon and nitrogen control during carbonitriding 32 6.3.3 Cold burn hazards 40 5.1.6 Internal oxidation 33 6.3.4 Pressurised piping and the gas expansion hazard 40 5.1.7 Hydrogen pickup 33 6.4 Control of safety hazards 40 5.1.8 Surface passivation 34 6.4.1 General safety regulations and guidelines 40 5.1.9 Gas analysis 34 6.4.2 Explosions/flammability/fire 40 5.1.9.1 Gas analysis with IR 34 6.5 Safe use of gases along the value chain 41 5.1.9.2 Oxygen probe 35 5.2 Automatic atmosphere composition control 35 References 42 06 Gas carburising and carbonitriding 1. Introduction. In order to achieve specific properties and the desired surface quality after heat treatment of a steel object, numerous process parameters need to be controlled. A most critical parameter is the composition, function and control of the furnace atmosphere. In carburising/ carbonitriding processes, the function of the atmosphere is to supply the necessary amount of carbon (and nitrogen) and to provide the right carbon (and nitrogen) content to the steel surface, and this is what determines the final properties of the carburised object. Therefore, it is important to ensure a reliable supply of required gases and process gas blends but also to integrate leading application technologies to enable precision control of furnace atmospheres and ultimately achieve the desired product specifications of steels. The purpose of this expert edition is to deliver a comprehensive technical overview of carburising and carbonitriding processes with critical influencing parameters in terms of the required equipment and furnace atmosphere. This expert edition should deliver valuable background information on a complex topic in a structured single document in order to achieve a higher confidence level in the readers’ business decisions. Each of the expert editions has a similar content structure. The first part focuses on the process, explaining the basic principles of the hardness and carbon content of steels as well as the properties of carburised steels. The next section focuses on the different types of furnaces and the required equipment in the process. The furnace atmosphere generation and required gas supply is highlighted in the fourth section; the interaction between furnace atmosphere and steel surface and how to control the atmosphere is described in the fifth section. As flammable, asphyxiating and toxic gases are used in carburising and carbonitriding processes, safety issues need to be addressed in the sixth section; this is an important concern of Linde Gas. Gas carburising and carbonitriding 07 2. Process. 68 1000 900 Carburising 65 900 800 Hard and brittle 800 700 60 700 600 600 Cooling 500 50 500 Temp [°C] Hardness HRC 400 Hardness Vickers 400 40 300 Tempering 200 300 30 20 100 200 10 0 0 02416 8 0 12 100 Time [h] 0 0.2 0.4 0.6 0.8 1.0 1.2 Weight% C Figure 1: Hardness as a function of carbon content in hardened steel. The shaded area Figure 2: Gas carburising cycle including the quenching and tempering steps shows the scatter effect of the retained austenite and alloy content of steel [1] 2.1 Basic principles The highest hardness of a hardened steel having a martensitic The terms carburising and carbonitriding are normally understood to microstructure is obtained when its carbon content is high, around 0.8 include hardening, and thus quenching, as the final step. In this step, weight% C (Figure 1). Steels with such high carbon content are hard, the carburised or carbonitrided case transforms to a hard martensite but also brittle, and therefore cannot be used in machine parts such microstructure constituent. as gears, sleeves and shafts that are exposed to dynamic bending and tensile stresses during operation. A carbon content as high as 1% C The term case hardening is sometimes alternatively used to more also makes the steel difficult to machine by cutting operations such as clearly describe the fact that the process includes the hardening step. turning or drilling. The process cycle shown in Figure 2 also includes tempering, which is required to ensure toughness by eliminating internal micro-stresses and These shortcomings can be eliminated by using a low carbon content by somewhat reducing the hardness. steel to machine a part to its final form and dimensions prior to carburising and hardening. The low carbon content in the steel A carburising atmosphere must be able to transfer carbon – and also ensures good machinability before carburising. After carburising and nitrogen in the case of carbonitriding – to the steel surface to provide quenching, the part will have a hard case due to the high surface carbon the required surface hardness. To meet hardness tolerance requirements, concentration, but a softer core, and this combination of properties will this transfer must result in closely controlled carbon or nitrogen assure good wear and fatigue resistance. The martensitic case attains a concentrations on the steel surface. hardness corresponding to its carbon content, as is shown in Figure 1. The case with increased carbon concentration is typically 0.1–1.5 mm The surface carbon concentration after carburising can, as indicated (0.004–0.060 inches) thick. The core of the part maintains its low carbon in Figure 3, be controlled by the ratio (vol.% CO)²/ (vol.% CO₂) in the concentration and corresponding lower hardness. furnace atmosphere. The atmosphere nitrogen activity, which plays an important role in carbonitriding, can be controlled by the ratio 3/2 In this expert edition, the term carburising is used for a heat treatment vol.% NH₃/ vol. % H₂ . Expressions for the atmosphere oxygen and process carried out at a temperature where the steel is austenitic, hydrogen activities are also shown in Figure 3. These are not of primary typically in the temperature range 820–950 °C (1510–1740 °F), and interest, but they are related to the oxidation risk for alloying elements which requires a controlled furnace atmosphere at slight overpressure and to hydrogen pickup respectively. that transfers carbon from the atmosphere to the steel surface. Similarly, the term carbonitriding is used where the aim is to transfer both carbon and nitrogen to the steel surface. 08 Gas carburising and carbonitriding Gas Gas/surface Metal N2 O2 CH4 H CO H2 O O NH3 N CO2 C C 3 H8 H2 2 PCO PCO · PH PCO Carbon activity or 2 or P P CO2 H2 O PO2 Figure 4: Example of charging equipment (Courtesy of Ipsen International GmbH) PCO2 PH2 O Oxygen activity or or PO2 PCO PH2 PNH3 Nitrogen activity or PN 3/2 2 PH2 Hydrogen activity P H2 Figure 3: Schematic illustration of atmosphere/metal interaction and expressions for proportionality between atmosphere composition and carbon and nitrogen activities.