Analysis of Surface Heat Treatment Methods for Castings

1、 Surface quenching
By rapidly heating the workpiece with different heat sources, and cooling it quickly when the surface temperature of the workpiece reaches the critical point or above (at which point the core temperature of the workpiece is below the critical point), the surface of the workpiece is hardened while the core maintains its original structure. In order to achieve the goal of heating only the surface of the workpiece, it is required that the heat source used has a high energy density. According to different heating methods, surface quenching can be divided into induction heating (high-frequency, medium frequency, power frequency) surface quenching, flame heating surface quenching, electrical contact heating surface quenching, electrolyte heating surface quenching, laser heating surface quenching, electron beam surface quenching, etc. The most commonly used methods in industry are induction heating and flame heating surface quenching.
2、 Chemical heat treatment
Place the workpiece in a medium containing active elements for heating and insulation, allowing the active atoms in the medium to penetrate the surface of the workpiece or form a coating of a certain compound, in order to change the structure and chemical composition of the surface, thereby giving the surface of the part special mechanical or physicochemical properties. Usually, other suitable heat treatments are required before and after chemical infiltration to maximize the potential of the infiltration layer and achieve the best match between the core and surface of the workpiece in terms of microstructure, properties, etc. According to the different infiltration elements, chemical heat treatment can be divided into carburizing, nitriding, boronizing, siliconizing, sulfidizing, aluminum infiltration, chromium infiltration, zinc infiltration, carbon nitrogen co infiltration, aluminum chromium co infiltration, etc.
3、 Contact resistance heating quenching
By applying a voltage of less than 5 volts to the workpiece through an electrode, a large current flows through the contact area between the electrode and the workpiece, generating a large amount of resistance heat, which heats the surface of the workpiece to the quenching temperature. Then, the electrode is removed, and the heat is transferred into the interior of the workpiece while the surface rapidly cools, achieving the purpose of quenching. When dealing with long workpieces, the electrode continuously moves forward, and the remaining part continuously hardens.
The advantages of this method are that the equipment is simple, the operation is convenient, it is easy to automate, the workpiece distortion is minimal, there is no need for tempering, and it can significantly improve the wear resistance and scratch resistance of the workpiece. However, the hardened layer is relatively thin (0.15-0.35mm). The uniformity of microstructure and hardness is poor. This method is mostly used for surface quenching of machine tool guides made of cast iron, and its application range is not wide.
4、 Electrolytic heating quenching
Place the workpiece in an electrolyte solution of acid, alkali, or salt, connect the workpiece to the cathode, and connect the electrolytic cell to the anode. After connecting to direct current, the electrolyte is electrolyzed, releasing oxygen on the anode and hydrogen on the workpiece. Hydrogen forms a gas film around the workpiece, becoming a resistor and generating heat. The surface of the workpiece is quickly heated to the quenching temperature, and then the power is cut off. The gas film immediately disappears, and the electrolyte becomes the quenching medium, rapidly cooling and hardening the surface of the workpiece. The commonly used electrolyte is an aqueous solution containing 5-18% sodium carbonate. The electrolytic heating method is simple, with a short processing time of only 5-10 seconds, high productivity, and minimal quenching distortion. It is suitable for large-scale production of small parts and has been used for surface quenching of engine exhaust valve stem ends.
5、 Laser heat treatment
The application research of laser in heat treatment began in the early 1970s, and then moved from the laboratory research stage to the production application stage. When a focused high-energy density (10W/cm) laser is irradiated onto a metal surface, the metal surface rises to the quenching temperature within a few hundredths or even thousandths of a second. Due to the rapid heating of the irradiation point, the heat cannot be transferred to the surrounding metal in time. Therefore, when the laser irradiation is stopped, the metal around the irradiation point acts as a quenching medium and absorbs a large amount of heat, rapidly cooling the irradiation point and obtaining an extremely fine structure with high mechanical properties. If the heating temperature is high enough to melt the metal surface, a smooth surface can be obtained after cooling, which is called polishing.
Laser heating can also be used for local alloying treatment, that is, coating a layer of wear-resistant or heat-resistant metal on the parts of the workpiece that are prone to wear or require heat resistance, or coating a layer of coating containing wear-resistant or heat-resistant metal, and then using laser irradiation to quickly melt it, forming a wear-resistant or heat-resistant alloy layer. Plating a layer of chromium on the parts that require heat resistance, and then quickly melting them with a laser to form a hard, tempering resistant, chromium containing heat-resistant surface layer, can greatly improve the service life and heat resistance of the workpiece.
6、 Electron beam heat treatment
Research and application began as early as the 1970s. In the early days, it was used for continuous annealing of thin steel strips and steel wires, with an energy density of up to 10W/cm. Electron beam surface quenching has the same characteristics as laser except that it should be carried out in vacuum. When an electron beam hits a metal surface, the impact point is rapidly heated. The depth at which an electron beam penetrates a material depends on the acceleration voltage and material density. For example, the theoretical penetration depth of a 150kW electron beam on an iron surface is approximately 0.076mm; On the surface of aluminum, it can reach 0.16mm.

The electron beam bombards the surface in a short period of time, causing a rapid increase in surface temperature while the substrate remains in a cold state. When the electron beam stops bombarding, heat quickly conducts to the cold base metal, causing the heated surface to self quench. In order to effectively carry out "self cooling quenching", a ratio of at least 5:1 should be maintained between the volume of the entire workpiece and the volume of the quenched surface layer. The surface temperature and quenching depth are also related to the bombardment time. Electron beam heat treatment has a fast heating speed, and the austenitization time is only a few tenths of a second or even shorter. Therefore, the surface grains of the workpiece are very fine, the hardness is higher than that of general heat treatment, and it has good mechanical properties.