In the last article, we introduced carbon carburizing in detail. In general, the carburizing is to heat the steel above to the critical temperature and then make the carbon infiltration and diffusion, so it is also called Austenitic chemical heat treatment. It is a kind of high-temperature chemical heat treatment, and the deformation of the workpiece is large. Generally, low carbon steel carburizing is used to obtain good core toughness and high surface hardness. After carburizing, the surface carbon content of the workpiece is generally higher than 0.8%. Quenching and tempering at low temperature, while improving the hardness and wear resistance, the heart can maintain high toughness, can withstand impact load, high fatigue strength. Carburizing steel can be divided into carbon carburizing steel and alloy carburizing steel. Alloy elements such as manganese, chromium, nickel, molybdenum, tungsten, vanadium, boron in the carburizing steel can improve the hardenability, grain refinement, strengthen the solid solution, affect the carbon content in the carburizing layer, the thickness of the carburizing layer and the organization.
Nitriding is the diffusion of nitrogen at a temperature lower than the eutectoid of iron and nitrogen, also known as low-temperature chemical heat treatment (ferrite chemical heat treatment)., it has a small deformation. When steel contains molybdenum, chromium, aluminum and other metal nitrides, can obtain higher hardness, wear resistance, corrosion resistance and fatigue resistance performance than carburized layer. Nitriding is mainly used for precision, distortion capacity, high fatigue strength and wear resistance are artifacts, such as boring spindle and boring bar, grinder spindle, cylinder liner, etc. Low and medium carbon alloy structural steels, tool steels, stainless steels and nodular cast iron containing Cr, Mo, V, Ti, Al and other elements can be nitrided.
Although nitrided steel has high hardness, wear-resistance and high fatigue strength, which only maintained on the surface (The nitrided layer of Cr-Mo-Al steel depth of 0.3–0.65mm at 500–540℃ after 35–65h ). Most of the nitriding parts work under friction and complex dynamic load conditions, where need both the surface and the core of the high performance. The nitriding carbon steel Fe 4N and Fe 2N is unstable, the temperature is slightly higher is easy to gather coarsening, the surface can not be higher hardness, and the core can not have a higher strength and toughness. In order to obtain high hardness and high wear resistance on the surface and core at the same time, it is necessary to form stable nitride compounds with nitrogen in the steel, which can strengthen the core alloy elements such as Al, Ti, V, W, Mo, Cr, etc. 41CrAlMo74 (SacM645/41CrAlMo7/34CrAlMo5) is a commonly used nitrided steel grade. Aluminum has a great affinity with nitrogen and is the main alloying element to form nitride and improve the strength of the nitriding layer. AlN is very stable, even at the temperature of about 1000C in the steel is insoluble and aluminum makes the steel has good nitriding performance, the hardness of this steel after nitriding surface can be as high as 1100–1200HV(67–72HRC).
In general, carburizing is a kind of metal surface treatment, nitriding is a chemical heat treatment process in a certain temperature and medium to make nitrogen atoms penetrate into the surface of the workpiece. Today’s article will introduce the difference between carburized steel and nitriding steel:
- Nitriding steel has better thermal stability than carburizing steel.
- Nitriding steel has higher surface hardness and wear resistance than carburizing steel After nitriding, the surface hardness of steel parts is as high as 1100-1200HV (equivalent to 67-72HRC), and the high hardness and wear resistance can be maintained to 560-600℃ without decreasing
- Nitriding steel has higher fatigue strength, lower occlusal resistance and lower notch sensitivity than carburizing steel. This is because the volume of the nitriding layer is enlarged and the residual compressive stress is formed in the surface layer.
- The corrosion resistance of nitriding steel is better than that of carburizing steel because of the formation of dense nitride film on the surface of steel parts.
- The nitriding temperature (500-600℃) is lower than carburizing temperature (900-1000℃).After nitriding, the steel does not need heat treatment, so the nitriding deformation is very small. However, the lower temperature makes nitriding slower and requires longer holding time than carburizing.
- Different applications. Nitriding steel is widely used in the mechanical industry, especially suitable for the final heat treatment of precision parts, such as grinding machine spindle, boring machine bar, precision machine screw, internal combustion engine crankshaft and various precision gears and measuring tools, etc. For low-impact applications that focus on speed, nitriding can provide a longer service life because of its higher surface hardness and better wear resistance. Carburized steel provides a longer service life in high-impact applications due to a deeper hardened layer and is widely used in aircraft, automobiles and tractors and other mechanical parts, such as gears, shafts, camshafts, etc.