H 13 is a widely used air-cooled quenching die steel. During the Eighth Five-Year Plan period, China focused on upgrading steel grades. H 13 steel has high toughness, cold and hot fatigue resistance, thermal fatigue crack resistance, strong adhesion resistance and little interaction with molten metal. Widely used in manufacturing hot upsetting, hot extrusion and die casting dies, especially suitable for die casting dies. The die castings produced by it have good appearance quality and long service life. At the same time, the steel has a high thermal strength, which is a kind of steel with high quality and low price and toughness, and is especially welcomed by die-casting die manufacturers.
As a die-casting die, the main factors affecting its life are: thermal fatigue (that is, cracking) will occur when the die is subjected to thermal cycle; In the process of die casting, molten metal is injected into the cavity mold, and the high-temperature metal is washed and the mold parts in direct contact will be eroded and corroded. Therefore, improving the thermal fatigue performance and corrosion resistance of the die is an effective way to improve the life of the H 13 steel die-casting die.
Because nitriding can improve the thermal fatigue and corrosion resistance of the die, the H 13 steel die-casting die was quenched and tempered, and then the compound heat treatment of nitriding was studied. It is found that the new technology can obviously improve the surface hardness, wear resistance, thermal fatigue resistance and corrosion resistance of H 13 steel die casting die, and the service life of the die is 1 times longer than that of Weitong heat treatment.
2 Mold materials and preheating treatment
(1) The material of the die-casting die is AISI-H 13, the overall dimension of the die is Ф 250 * 1 20mm, and the hardness of the matrix is required to be 46 ~ 48 HRC. See table1for its chemical composition and critical temperature.
Table 1 H 13: chemical composition and critical temperature (℃)ASTMA68 1-94.
The critical temperature (%) of chemical composition of steel grade is new (℃).
csimncrmovpsac 1ac 3 ar 1ar 3 ms
h 130.32-0.450 . 80- 1.200 . 20-0.504 . 75-5.2 1. 10- 1.750 . 80- 1.20≤0.03≤0.03850 9 10700820335
(2) Pretreatment process.
The pretreatment of H 13 steel was carried out in a high temperature salt bath furnace. In order to reduce the thermal stress of H 13 steel die casting die and promote austenite homogenization, preheating should be carried out in stages before reaching the austenitizing temperature. Therefore, before entering the salt bath furnace, two-stage preheating of 550℃ 40 min and 850℃ 40 min should be adopted, and the optimal salt heating temperature is 65438 0030℃.
Nitriding and carbonitriding heat treatment of 3 H 13 steel die casting die.
Nitrogen-carbon infiltration is selected for H 13 steel die casting die because the process is carried out in the low temperature range (500-600℃), and the solid phase depth of carbon atoms in α iron is lower than that of nitrogen atoms. Due to the mutual promotion of nitrogen atoms and carbon atoms, the infiltration speed of nitrogen and carbon atoms is greatly improved. Considering that nitrocarburizing will not affect the matrix hardness of H 13 steel die casting die, the nitrocarburizing temperature should be lower than the tempering temperature.
3. 1 equipment
The nitrocarburizing of H 13 steel die casting die can be carried out in a 75kW gas nitriding furnace.
3.2 nitrocarburizing process.
The nitrocarburizing heat treatment of H 13 steel die casting die is carried out in the production of conventional gas nitrocarburizing heat treatment. Before nitrocarburizing, the final finishing (grinding, EDM, etc.) is carried out. ) must be carried out on the die-casting die of H 13 steel after preliminary heat treatment. Degrease with gasoline or alcohol before charging the furnace. After cleaning, the working surface should be free of rust or dirt.
The nitrocarburizing medium of H 13 steel die casting die is ammonia+ethanol, and the process is 580℃×4.5h ..
3.3 Microstructure and properties after nitrocarburizing
(1) permeable layer structure.
The microstructure of H 13 steel after nitrocarburizing is Fe2N, Fe3N, Fe4N and alloy nitrides such as C, Mo and V in turn.
(2) Thickness and surface hardness of nitrided layer.
After nitrocarburizing, the thickness of H 13 sample is about 0.20 mm, the surface hardness is more than: 900HV, and the wear resistance is high. Compared with quenched and tempered samples and high-frequency quenched samples, the wear weight loss can be reduced by 1-2 orders of magnitude or doubled.
(3) Fatigue resistance.
After nitrocarburizing, because the surface is in a state of compressive stress, γ′ and other dispersed precipitates hinder dislocation slip, and the fatigue limit of nitrocarburizing is 25%-35% higher than that of carburizing and high frequency quenching.
From the quenching, tempering and nitrocarburizing results of H 13 steel die casting die, the following conclusions can be drawn:
(1)H 13 steel die casting die is quenched and tempered, and then nitrocarburizing is carried out, so that higher surface hardness, wear resistance, fatigue resistance and corrosion resistance can be obtained. At the same time, gas nitrocarburizing is equivalent to the first tempering after die quenching, and the die deformation is small.
(2) Nitriding gas nitrocarburizing has the characteristics of short production cycle, low temperature, simple equipment and convenient operation.
(3) The production practice shows that the service life of H 13 steel die casting die after quenching and tempering treatment is more than twice that of conventional heat treatment.
(4) (4) The better heat treatment process for die-casting carbon of H13 steel is: 1030℃ quenching, 600℃ tempering, die repair first, then nitrocarburizing at 580℃+4.5h, and oil cooling after nitriding.
reference data
Tianya community. Tianya community [reference time 2018-4-11]