We introduced the method of nitriding steel in the last article. As we know, nitriding treatment refers to the chemical heat treatment process in which nitrogen atoms penetrate into the surface of the workpiece under a certain temperature and medium to increase the wear resistance, surface hardness, fatigue limit and corrosion resistance of steel parts. Nitriding metal products have excellent wear resistance, fatigue resistance, corrosion resistance and high-temperature resistance. The elements of aluminum, chromium, vanadium and molybdenum in steel contribute to nitriding, which forms stable nitride compounds when they come into contact with primitive nitrogen atoms at nitriding temperature.
Molybdenum, in particular, not only acts as a nitride forming element, but also reduces the brittleness at the nitriding temperature. Generally speaking, if the steel contains one or more nitride-forming elements, the nitriding effect is better, in which 0.85 — 1.5% aluminum is the best nitriding result, a certain amount of chromium content can also get a good effect. Plain carbon steel is not suitable for nitriding steel because the nitriding layer is brittle and easy to peel off.
Commonly used nitriding steels including:
(1) Low alloy steels containing aluminum (Standard nitriding steels)
(2) Cr-containing medium carbon low alloy steel SAE 41XX series, 4300, 5100, 6100, 8600, 8700, 9800 series.
(3) Hot work die steel (Containing about 5% chromium), SAE H11 (SKD — 61) H12, H13
(4) Ferrite and Martensitic stainless steel SAE 400 system
(5) Austenitic stainless steel SAE 300 series
(6) Precipitate hardened stainless steel 17-4PH, 17-7PH, A-286, etc
Standard nitriding steels containing aluminum have a high hardness and wear-resistant surface layer after nitriding, but the hardened layer is also brittle. On the contrary, low alloy steels containing chromium have lower hardness, but the hardened layer is more ductile, and the surface also has considerable wear resistance and beam resistance. Therefore, the characteristics of materials and parts should be combined when choosing materials. Tool steels such as H11 (SKD61), D2 (SKD — 11) have high surface hardness and high core strength.
Nitriding process of low alloy steels containing aluminum
Surface cleaning of parts before Nitriding
Most of the parts can be immediately nitriding after gas oil, some parts can also be cleaned with gasoline, but polishing, grinding, polishing and other processing methods may hinder the nitriding surface layer, resulting in uneven nitriding layer or bending and other defects. At this time, the following methods should be used to remove the surface layer. Firstly, before nitriding, gas was used for the oil removal, then aluminum oxide powder was used for the sandblasting treatment on the surface, and then phosphate coating on the surface was processed.
Discharge air in nitriding furnace
The treated parts can be heated after sealing in the nitriding furnace, but the air must be removed from the furnace before heating to 150℃. Excluding the gas in the furnace is mainly to prevent ammonia decomposition and air contact and explosive gas, and to prevent the surface oxidation of the treated material and support. The gases used are ammonia and nitrogen. The process is as follows: (1) After the processed parts furnace cover sealed, began to flow into the anhydrous ammonia (2) To set the automatic temperature control of reheating furnace at 150 ℃ and start heating furnace (note the furnace temperature higher than 150 ℃) (3) The air out of below 10%, or discharge of gas containing more than 90% of NH3, furnace temperature rise again to nitriding temperatures.
The decomposition rate of ammonia
Nitriding is carried out in contact with other alloying elements and primary nitrogen, that is, the steel itself becomes a catalyst when the ammonia gas is in contact with the heating steel and promotes the decomposition of ammonia. Although in different decomposition rate of ammonia can be nitrided, but generally use 15 — 30% of the decomposition rate, and according to the required thickness of nitriding to maintain at least 4 — 10 hours, the treatment temperature is maintained at about 520℃.
Most industrial nitriding furnaces are combined with a heat exchanger to cool the furnace and the parts to be treated after the nitriding is completed. That is, after the nitriding is completed, turn off the heating power supply to reduce the furnace temperature by about 50℃, and then double the flow of ammonia to start the heat switch. At this time, it is necessary to observe whether there is bubble overflow in the glass bottle connected to the exhaust pipe to confirm the positive pressure in the furnace. After the ammonia in the furnace stabilized, immediately reduce the flow of ammonia to maintain the positive pressure. When the furnace temperature drops below 150℃, use the method of discharging the gas in the furnace, and open the furnace cover after introducing air or nitrogen.