Carburizing and nitriding generally refer to chemical heat treatment of surface of steel.

Carburizing must use low carbon steel or low carbon alloy steel, which can be divided into three types: solid, liquid and gas carburizing. Widely used gas carburizing, heating temperature 900-950℃. Carburizing depth mainly depends on holding time, which is generally estimated at 0.2-0.25 mm per hour. Surface carbon content can reach 0.85-1.05 percent. After carburizing, it must be heat treated, it is usually tempered at low temperature after quenching to obtain wear-resistant and impact-resistant parts with high surface hardness and high core toughness.
Carburizing of steel is to heat low-carbon steel in a carbon-rich medium to a high temperature (generally 900-950℃), so that activated carbon atoms penetrate into surface of steel to obtain a high-carbon carburized structure. After quenching and tempering at low temperature, surface has high hardness, wear resistance and fatigue resistance, while heart still maintains sufficient strength and toughness.

The most widely used gas nitriding, heating temperature is 500-600 degrees Celsius. Nitrogen atoms form nitrides with aluminum, chromium, and molybdenum on the surface of steel. General depth is 0.1-0.6 mm. Nitrided layer can obtain high hardness without quenching. This performance can be maintained to 600-650 degrees Celsius. Deformation of workpiece is small, which can prevent corrosion of water vapor and alkaline solutions, but production cycle is long, cost is high, nitrided layer is thin and brittle, and it is not suitable to bear concentrated heavy loads. Mainly used for processing important and complex precision parts.
Coating are physical methods, and "permeation" is a chemical change, which is essentially different.

 

Generally in the range of 0.15-0.25%. For heavy-duty cementite, it can be increased to 0.25-0.30%, so that core still has sufficient plasticity and toughness after quenching and low temperature tempering. But carbon content cannot be too low, otherwise, a certain strength cannot be guaranteed.

Improve hardenability, refine grains, strengthen solid solution, affect carbon content, thickness and structure of infiltration layer. Alloying elements usually added in carburizing steel are manganese, chromium, nickel, molybdenum, tungsten, vanadium, boron and so on.
Commonly used carburizing steel can be divided into two types of carburizing steel and alloy carburizing steel.

The most commonly used are 15 and 20 steels, whose surface hardness can reach 56-62HRC after carburizing and heat treatment. However, due to its low hardenability, it is only suitable for small parts such as shaft sleeves, chains that have low core strength requirements, low stress, and wear resistance.

Such as 20Cr, 20Cr2MnVB, 20Mn2TiB, etc., their permeability and core strength are higher than carbon carburizing steel, can be used to make more important carburizing parts in general machinery, such as gears, piston pins in automobiles and tractors.

Such as 20Cr2Ni4, 18Cr2N4W, 15Si3MoWV, etc., due to their high hardenability, high strength and toughness, they are mainly used to manufacture parts with large cross-sections, heavy loads, and complex forces, such as aero engine gears and shafts .
Solid carburizing, liquid carburizing, and gas carburizing, carburizing temperature is 900-950℃, surface layer (carbon) is 0.8-1.2%, and layer depth is 0.5-2.0mm.

Carburizing workpieces should actually be regarded as a kind of composite material with very different surface and center contents. Carburizing can only change carbon content of surface of workpiece, final strengthening of surface and core must be achieved through appropriate heat treatment. After carburizing, workpieces need to be quenched and tempered at low temperature.
Purpose of quenching is to form high-carbon martensite or high-carbon martensite and fine-grained carbide structures on the surface. Low temperature tempering temperature is 150-200℃.

(1) Pre-treatment normalization before carburizing is to improve original structure of material, reduce band shape, eliminate Weiss structure, make surface roughness fine, and eliminate unreasonable state of material flow line. Normalizing process: 860-980℃ air-cooled, 179-217HBS.
(2) Hardness of workpiece to be machined after carburizing should not be higher than 30HRC.
(3) For carburized and hardened parts with thin-walled grooves, thin-walled grooves cannot be processed before carburizing.
(4) Carburizing shall not be prevented by galvanizing.

(1) Increase margin method. A certain machining allowance is reserved in advance at parts that do not require carburizing, and remainder is more than double depth of carburizing layer. After carburizing, carburized layer is first removed and then quenched.
(2) Copper plating method. A layer of copper with a thickness of 0.02-0.04mm is electroplated at parts that do not require carburizing.
(3) Coating method. Apply impervious coatings on areas where carburization is not required.
(4) Tooling method. Self-made special tooling to seal and seal parts that do not need carburizing.

A chemical heat treatment process in which nitrogen atoms penetrate into surface of steel to form a nitrogen-rich hardened layer.
Compared with carburizing, parts after nitriding have high hardness, wear resistance, high fatigue strength, higher seizure resistance and higher corrosion resistance. Nitriding process is performed below phase transition temperature of steel (450-600℃), so deformation is small and volume is slightly expanded. Disadvantages are long cycle (nitriding time of general gas nitriding process is up to several 10h to 100h), high cost, thin nitrided layer (usually about 0.5mm), and brittleness, which cannot bear too much contact stress and impact load.

In theory, all steel materials can be nitrided. However, we only refer to those steels that can be treated with nitriding and can obtain satisfactory results as nitriding steel. All low- and medium-carbon alloy structural steels, tool steels, stainless steels (Before nitriding of stainless steel, passivation film on the surface of workpiece needs to be removed, stainless steel and heat-resistant steel can be directly treated with ion nitridation.) and ductile cast irons containing elements such as Cr, Mo, V, Ti, Al can be nitriding.
Although parts after nitriding have high hardness, high wear resistance and high fatigue strength, they are only a thin layer (Cr-Mo-Al steel at 500-540℃, after 35-65h, nitriding layer depth is only 0.3-0.65mm). A strong and tough heart tissue must be used as solid base of nitriding layer in order to play the most important role of nitriding.
In general, most nitriding parts work under conditions of friction and complex dynamic loads, performance of both surface and core is very high.
If carbon steel is used for nitriding, Fe4N and Fe2N formed are relatively unstable. If temperature is slightly higher, it is easier to aggregate and roughen, it is impossible to obtain higher hardness on the surface, core cannot have higher strength and toughness. In order to obtain high hardness and high wear resistance on the surface, at the same time to obtain a strong and tough heart structure, it is necessary to add to steel on one hand that it can form stable nitrides with nitrogen and also strengthen core alloy elements, such as Al, Ti, V, W, Mo, Cr, etc., which can form stable compounds with nitrogen. Among them, Cr, W, Mo, V can also improve structure of steel, increase strength and toughness of steel.
At present, steel used for nitriding is 38CrMoAlA. Among them, aluminum has a great affinity with nitrogen and is main alloying element for forming nitrides to improve strength of nitrided layer. AlN is very stable and does not dissolve in steel to a temperature of about 1000℃. Due to effect of aluminum, steel has good nitriding properties. After nitriding, steel has a surface hardness of 1100-1200HV(equivalent to 67-72HRC). 38CrMoAlA steel has a serious tendency of decarburization, each process must leave a large machining allowance.
For nitrided parts with high hardness and high wear resistance, carbon steel and general alloy steel are not suitable. For nitrided parts whose main purpose is to improve corrosion resistance, carbon steel and general alloy steel can be selected.

(1) Pretreatment heat treatment and tempering before nitriding. Before nitriding workpiece, it should be quenched and tempered to obtain tempered sorbite structure. Tempering temperature is generally higher than nitriding temperature.
(2) Pre-heat treatment and de-stress treatment before nitriding. Before nitriding, internal stress generated during machining should be eliminated as much as possible to stabilize size of part. Stress relief temperature should be lower than tempering temperature, holding time should be longer than tempering time, and then slowly cool to room temperature. Parts with larger section sizes should not be normalized. Tool steel must be quenched, tempered, and not annealed.
(3) Surface roughness Ra of nitriding parts shall be less than 1.6um, surface shall be free from defects such as burrs, bumps and rust. Parts that cannot be processed in time must be protected with oil to prevent rusting. When hoisting into furnace, clean it with clean gasoline to ensure cleanliness.
(4) For workpieces with sharp corners and sharp edges, nitriding is not suitable.
(5) For protection of local non-nitriding parts, it is not appropriate to use method of leaving a machining allowance.