Carburizing must be made of low carbon steel or low carbon alloy steel, which can be divided into solid, liquid and gas carburizing. A wide range of gas carburizing is used, and the heating temperature is 900-950 °C. Carburizing depth is mainly determined by the holding time and is generally estimated to be 0.2-0.25 mm per hour. The surface carbon content can reach 0.85-1.05 percent. After carburizing, heat treatment must be carried out. After quenching, it is usually tempered at low temperature to obtain wear-resistant and impact-resistant parts with high surface hardness and high toughness of the core.
Carburizing of steel is to heat the low carbon steel to a high temperature (generally 900-950 ° C) in a carbon-rich medium, so that activated carbon atoms penetrate into the surface of the steel to obtain a high carbon layer structure. After quenching and low temperature tempering, the surface has high hardness, wear resistance and fatigue resistance, while the core still maintains sufficient strength and toughness.
2, nitriding
Apply a wide range of gas nitriding, heating temperature of 500-600 degrees Celsius. The nitrogen atom forms a nitride with aluminum, chromium and molybdenum in the surface of the steel, and the depth is generally 0.1-0.6 mm. The nitrided layer can be obtained with high hardness without quenching, and the performance can be maintained to 600-650 degrees Celsius. The deformation of the workpiece is small, which can prevent the corrosion of water vapor and alkaline solution, but the production cycle is long and the cost is high. The nitride layer is thin and brittle, and it is not suitable to bear the concentrated heavy load. Mainly used to handle important and complex precision parts.
Coating and coating are physical methods, while "osmosis" is a chemical change that is fundamentally different.
First, the chemical composition of carburized steel
(1) Carbon content of carburized steel
Generally in the range of 0.15-0.25%. For heavy-duty cementite, it can be increased to 0.25-0.30% so that the core still has sufficient plasticity and toughness after quenching and low temperature tempering. However, the carbon content cannot be too low, otherwise, a certain strength cannot be guaranteed.
(2) The role of alloying elements in carburizing steel
Improve hardenability, refine grains, strengthen solid solution, affect carbon content, thickness and microstructure of the layer. The alloying elements usually added to the carburized steel are manganese, chromium, nickel, molybdenum, tungsten, vanadium, boron, and the like.
Commonly used carburized steel can be divided into two categories: carbon carburized steel and alloy carburized steel.
(1) Carbon carburized steel
The 15 and 20 steels are used more, and after surface carburization and heat treatment, the surface hardness can reach 56-62HRC. However, due to the low hardenability, it is only suitable for small parts with low core strength requirements, low force and wear, such as bushings and chains.
(2) Low alloy carburizing steel
For example, 20Cr, 20Cr2MnVB, 20Mn2TiB, etc., its permeability and core strength are higher than carbon carburized steel, and can be used to manufacture more important carburizing parts in general machinery, such as gears and piston pins in automobiles and tractors.
(3) Medium alloy carburized steel
Such as 20Cr2Ni4, 18Cr2N4W, 15Si3MoWV, etc., due to its high hardenability and high strength and toughness, it is mainly used to manufacture parts with large cross-section, heavy bearing and complicated force, such as gears and shafts of aero-engines. .
Solid carburizing, liquid carburizing, gas carburizing, carburizing temperature is 900-950 ° C, surface layer (carbon) is 0.8-1.2%, layer depth is 0.5-2.0 mm.
Second, heat treatment after carburizing
The carburized part should actually be regarded as a composite material with a very different surface and center content. Carburizing can only change the carbon content of the surface of the workpiece, and the final strengthening of the surface and the core must be achieved by appropriate heat treatment. The carburized workpiece needs to be quenched and tempered at low temperature.
The purpose of quenching is to form high carbon martensite or high carbon martensite and fine grained carbide structure on the surface. The low temperature tempering temperature is 150-200 °C.
Third, carburizing parts considerations
(1) Pre-normalization prior to carburizing, the purpose is to improve the original structure of the material, reduce the band shape, eliminate the Wei's structure, make the surface roughness thinner, and eliminate the unreasonable state of the material flow line. Normalizing process: air cooling with 860-980 °C, 179-217HBS.
(2) For workpieces that need to be machined after carburizing, the hardness should not exceed 30HRC.
(3) For carburized and quenched parts with thin-walled grooves, the thin-walled grooves cannot be processed before carburizing.
(4) Do not use galvanizing to prevent carburization.
Fourth, to prevent carburizing methods
(1) Increase the margin method. A certain machining allowance is reserved in advance in the portion where no carburization is required, and the retention is more than doubled than the depth of the carburized layer. After carburizing, the car is first removed to the carburizing layer and then quenched.
(2) Copper plating method. A layer of 0.02-0.04 mm of copper is plated on the part where no carburization is required. The copper layer should be dense and the original metal should not be exposed.
(3) Coating method. Apply anti-seepage coating to areas where no carburization is required.
(4) Tooling method. Self-made special tooling to seal the parts that do not need carburization.
5. Nitriding of steel (enhanced nitriding and anti-corrosion nitriding)
A chemical heat treatment process in which nitrogen atoms are infiltrated into the surface of steel to form a nitrogen-rich hardened layer.
Compared with carburizing, the parts after nitriding have high hardness and wear resistance, high fatigue strength, high anti-seizure, and high corrosion resistance. The nitriding process is carried out below the phase transition temperature of the steel (450-600 ° C), so that the deformation is small and the volume is slightly expanded. The disadvantage is that the cycle is long (the nitriding time of the gas nitriding process is as long as 10h to 100h), the cost is high, the layer is thin (generally about 0.5mm) and brittle, and it cannot withstand too much contact stress and impact load.
6. Steel for nitriding
In theory, all steel materials can be nitrided. However, we only refer to steels that are suitable for nitriding and can achieve satisfactory results. Low and medium carbon alloy structural steel, tool steel, stainless steel containing elements such as Cr, Mo, V, Ti, Al, etc. (The passivation film of the surface of the workpiece needs to be removed before nitriding. The stainless steel and heat resistant steel can be directly used for ions. Nitriding can be performed by nitriding method, ductile iron, and the like.
Although the parts after nitriding have high hardness, high wear resistance and high fatigue strength, they are only a thin layer (chromium molybdenum aluminum steel at 500-540 ° C, and the depth of nitriding layer is only 0.3 by 35-65h). -0.65mm). A strong and tough heart tissue must be used as a solid base for the nitriding layer to exert a greater effect on nitriding.
In general, most of the nitriding parts work under friction and complex dynamic loads, regardless of the surface and core properties.
If nitriding with carbon steel, the formed Fe4N and Fe2N are less stable. When the temperature is slightly higher, it is easy to aggregate and roughen, the surface is unlikely to have higher hardness, and the core cannot have higher strength and toughness. In order to obtain high hardness and high wear resistance on the surface, and to obtain a strong and tough core structure, it is necessary to add an alloying element such as Al, which can form a stable nitride with nitrogen and strengthen the core. Ti, V, W, Mo, Cr, etc. can form stable compounds with nitrogen. Among them, Cr, W, Mo, V can also improve the microstructure of steel and improve the strength and toughness of steel.
At present, the steel used exclusively for nitriding is 38CrMoAlA, in which aluminum has a great affinity with nitrogen and is the main alloying element for forming nitride to increase the strength of the nitrided layer. AlN is very stable and does not dissolve in steel at temperatures up to about 1000 °C. Due to the action of aluminum, the steel has good nitriding performance. The steel is nitrided and the surface hardness is as high as 1100-1200 HV (equivalent to 67-72HRC). 38CrMoAlA steel has a serious decarburization tendency, and each process must have a large processing allowance.
For nitriding parts requiring high hardness and high wear resistance, carbon steel and general alloy steel are not suitable. For the nitrided parts mainly for improving the corrosion resistance, carbon steel and general alloy steel can be selected.
Seven, nitriding parts precautions
(1) Preheating and quenching and tempering before nitriding. The nitriding workpiece should be quenched and tempered before nitriding to obtain tempered sorbite structure. Quenching and tempering treatment tempering temperature is generally higher than nitriding temperature.
(2) Pre-heat treatment before nitriding. Stress-relieving treatment. Before nitriding, the internal stress generated during machining should be eliminated as much as possible to stabilize the part size. The stress-relieving temperature should be lower than the tempering temperature, the holding time is longer than the tempering time, and then slowly cooled to room temperature. Parts with large cross-sections should not be normalized. The tool steel must be quenched and tempered, and no annealing is required.
(3) The surface roughness Ra of the nitriding parts shall be less than 1.6um, and the surface shall not have defects such as napping, bumping and rusting. Parts that cannot be disposed of in time must be oiled to prevent rust. When hoisting into the furnace, clean it with clean gasoline to ensure cleanliness.
(4) Workpieces with sharp corners and sharp edges are not suitable for nitriding.
(5) The protection of the local non-nitriding part, it is not appropriate to use the method of leaving the machining allowance.
(6) The workpiece whose surface has not been ground shall not be nitrided.