Can stainless steel be nitrided?

Nitriding treatment refers to the chemical heat treatment process in which nitrogen atoms penetrate into the surface of the workpiece at a certain temperature and in a certain medium. Nitriding products have excellent wear resistance, fatigue resistance, corrosion resistance and high temperature resistance.

brief introduction

The elements of aluminum, chromium, vanadium and molybdenum in traditional alloy steel are very helpful for nitriding. When these elements come into contact with nitrogen atoms in the primary state at nitriding temperature, stable nitrides are produced. In particular, molybdenum is use not only as an element for generating nitride, but also as an element for reduce brittleness at nitriding temperature. Elements in other alloy steels, such as nickel, copper, silicon and manganese, do not contribute much to nitriding characteristics. Generally speaking, if steel contains one or more nitride generating elements, the effect after nitriding is better. Among them, aluminum is the strongest nitriding element, and 0.85 ~ 1.5% aluminum has the best nitriding effect. In the aspect of chromium-containing chromium steel, if there is enough content, good results can also be obtained. But there is no carbon steel containing alloy, because the nitrided layer is brittle and easy to peel off, so it is not suitable for nitrided steel.

There are six commonly used nitriding steel:

(1) Low alloy steel containing aluminum (standard nitrided steel)

(2) SAE 4 100, 4300, 5 100, 6 100, 8600, 8700 and 9800 series of chromium-containing medium carbon low alloy steels.

(3) Hot-working die steel (containing about 5% chromium) SAE H11(SKD–61) H12, H 13.

(4) ferritic and martensitic stainless steel SAE 400 series

(5) Austenitic stainless steel SAE 300 series

(6) precipitation hardened stainless steel 17-4PH, 17-7PH, A-286, etc.

Standard nitriding steel containing aluminum can obtain a high hardness and wear-resistant surface layer after nitriding, but its hardened layer is also very brittle. On the contrary, the hardness of low alloy steel containing chromium is lower, but the toughness of hardened layer is better, and its surface also has considerable wear resistance and core resistance. Therefore, when selecting materials, we should pay attention to the characteristics of materials and make full use of their advantages to meet the functions of parts. Tool steels such as H11(SKD61) D2 (SKD–11) have high surface hardness and high core strength.

Technological process

Surface cleaning of parts before nitriding

Most parts can be nitrided immediately after gas degreasing. Some parts need to be cleaned with gasoline, but if polishing, grinding and polishing are used in the final processing method before nitriding, the surface layer that hinders nitriding may be produced, resulting in uneven nitriding layer or bending defects after nitriding. At this point, one of the following two methods should be used to remove the surface layer. The first method is to remove oil with gas before nitriding. And then grinding and cleaning the surface with alumina powder. The second method is to coat the surface with phosphate.

Removing air from nitriding furnace

Put the parts to be processed into the nitriding furnace, seal the furnace cover and heat them, but before heating to 150℃, the air in the furnace must be removed.

The main function of the exhaust furnace is to prevent ammonia gas from contacting with air during decomposition to produce explosive gas, and to prevent the surface oxidation of the treated object and support. The gases used are ammonia and nitrogen.

The main points of eliminating air in the furnace are as follows:

(1) After the processed parts are installed, seal the furnace cover and introduce anhydrous ammonia, with the maximum flow rate.

② Set the automatic temperature control of the heating furnace to 150℃ and start heating (note that the furnace temperature cannot be higher than 150℃).

(3) When the air in the furnace is discharged below 10%, or the discharged gas contains more than 90% NH3, raise the furnace temperature to the nitriding temperature.

Decomposition rate of ammonia

Nitriding is carried out by diffusing other alloying elements in contact with nascent state nitrogen, but the generation of nascent state nitrogen means that when ammonia gas contacts heated steel, steel itself becomes a catalyst, which promotes the decomposition of ammonia.

Although nitridation can be carried out under ammonia gas with different decomposition rates, the decomposition rate is generally 15 ~ 30%, and the required thickness for nitridation is kept at least 4 ~ 10 hour, that is, the treatment temperature is kept at about 520℃.

cool

Most industrial nitriding furnaces have heat exchangers to quickly cool the heating furnace and the parts to be treated after nitriding. That is, after nitriding is completed, the heating power supply is turned off to reduce the furnace temperature by about 50℃, then the ammonia gas flow rate is increased and the heat exchanger is started. At this time, it is necessary to observe whether there are bubbles overflowing from the glass bottle connected with the exhaust pipe to confirm the positive pressure in the furnace. After the ammonia introduced into the furnace is stable, the flow rate of ammonia can be reduced until the positive pressure in the furnace is maintained. When the furnace temperature drops below 150℃, the furnace cover can only be opened after air or nitrogen is introduced by the above-mentioned method of removing gas from the furnace.

Gas nitriding

The gas nitriding system was published by AF ry in Germany at 1923. The workpiece is placed in a furnace, and NH3 gas is directly transported into a nitriding furnace at 500-550℃ for 20-100 hours, so that NH3 gas is decomposed into atomic (N) gas and atomic (H) gas for nitriding treatment, and a wear-resistant and corrosion-resistant compound layer is formed on the surface of steel. Its thickness is about 0.02 ~ 0.02 m/m, its nature is extremely hard HV 1000 ~ 1200, and it is extremely brittle. The decomposition rate of NH3 varies with flow rate and temperature. The greater the flow rate, the lower the decomposition rate, the greater the flow rate, the higher the decomposition rate and the lower the temperature.

NH3 →〔N〕Fe + 3/2 H2

The decomposed N diffuses to the surface of steel to form. The common disadvantages of phase Fe2-3N gas nitriding are thin hardened layer and long nitriding time.

Because of the decomposition of NH3, the nitriding efficiency of gas nitriding is low, so steel grade suitable for nitriding is generally selected, such as containing nitriding elements such as Al, Cr and Mo, otherwise nitriding can hardly be carried out. Generally, JIS, SACM 1, new JIS, SACM645, SKD6 1 are used for toughening treatment, which is also called tempering. Al, Cr and Mo are all elements that increase the abnormal temperature, so the quenching temperature is high. Because the feed pipe and screw of plastic injection molding machine have rough surface, high hardness and brittleness, and long-term grinding is uneconomical, NH3 gas nitriding is adopted.

Liquid nitriding method

The main difference of liquid soft nitriding is that there is Fe3Nε phase, Fe4Nr phase but no Fe2Nξ phase nitride in the nitriding layer, and the ξ phase compound is hard and brittle, which is a nitride with poor toughness in nitriding treatment. The method of liquid soft nitriding is to remove rust, degrease and preheat, then put it into a nitriding crucible, with TF- 1 as the main salt, and heat it to 560 ~. The depth of nitrided layer is determined according to the external load on the workpiece. During the treatment, an air pipe must be introduced into the bottom of the crucible to decompose a certain amount of air nitriding agent into CN or CNO, which permeates and diffuses to the working face, so that the outermost compound on the surface of the workpiece is 8-9 wt% N and a small amount of C and diffusion layer, and nitrogen atoms diffuse into α-Fe base, making the steel part more fatigue-resistant. During nitriding, due to the decomposition and consumption of CNO, it will be continuously reduced to 6-8.

The material used in liquid soft nitriding is iron metal, and the surface hardness after nitriding is higher with al, Cr, Mo and Ti elements. The more gold content, the shallower the nitriding depth, such as carbon steel Hv 350~650, stainless steel HV 1000 ~ 1200, and nitrided steel HV 800 ~165430.

Liquid soft nitriding is suitable for wear-resistant and fatigue-resistant automobile parts, sewing machines, cameras and so on. , such as cylinder liner treatment, valve treatment, piston tube treatment and die parts that are not easily deformed. Countries adopting liquid soft nitriding, western European countries, the United States, the Soviet Union, Russia and Japan.

ionitriding

Put the workpiece into a nitriding furnace, vacuumize the furnace to 10-2 ~ 10-3 Torr (㎜ Hg) in advance, introduce N2 gas or mixed gas of N2+H2, adjust the furnace to 1 ~ 10 Torr, connect the furnace body to the anode, and connect the workpiece. At this time, N2 gas in the furnace will glow and discharge into positive ions, and the positive ions will move to the working face, and the cathode voltage will drop sharply in an instant, so that the positive ions will rush to the cathode surface at high speed, and the kinetic energy will be converted into gas energy, which will increase the surface temperature of the workpiece. After nitrogen ion impact, iron, carbon, oxygen and other elements. It will splash out of the surface of the workpiece and combine with nitrogen ions to form FeN, so that iron nitride will gradually adsorb on the workpiece and produce nitriding. Nitrogen is basically used in ion nitriding, but it can also be treated by ion soft nitriding if hydrogen carbide gas is added, but it is generally called ion nitriding. The nitrogen concentration on the workpiece surface can be adjusted by changing the partial pressure ratio of the mixed gas (N2+H2) filled in the furnace. During pure ion nitriding, the nitrogen content of single-phase R ′ (Fe4N) structure on the working face is 5.7 ~ 6. 1% wt, and the thick layer reaches 60%. This composite layer is tough rather than porous, and it is not easy to fall off. Because the iron nitride is absorbed by the workpiece and diffused to the inside, the structure changes orderly from the surface to the inside, and the single-phase ε(Fe3N) contains 5.7 ~ 1 1.0% wt. Single phase ξ(Fe2N) contains11.0 ~11.35% wt. Ion nitriding forms R phase, and after adding hydrogen carbide system, it becomes ε phase compound layer and diffusion layer. The increase of diffusion layer contributes to the improvement of fatigue strength. The ε phase is the most corrosive.

The degree of ion nitriding can start from 350℃. Considering the choice of materials and their related mechanical properties, the treatment time can range from a few minutes to a long time. This method is different from the previous thermal decomposition nitriding treatment method. Because of the high ion energy, this method can simply perform excellent surface hardening treatment on materials that were difficult to treat in the past, such as stainless steel, titanium and cobalt.