Heat treatment of pyrotechnic iron-based powder me

2022-08-07
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The interaction between materials and biological organism is mainly manifested in two aspects> heat treatment of iron-based powder metallurgy parts

Abstract: heat treatment is a mature and frequently used process technology. This paper reviews the effect of porosity and alloy content on the hardenability of iron-based P/M parts when they are integrally quenched

key words: iron based powder metallurgy parts; Heat treatment; Hardenability

in the production of iron-based P/M parts, many properties and structures that the part materials must have are formed in the sintering process, but some of them can be improved and perfected only through subsequent heat treatment. Therefore, heat treatment is a very important technology for iron-based powder metallurgy parts industry

although the heat treatment principle of iron-based P/M parts is the same as that of cast and forged parts with the same composition, the P/M parts may have a certain amount of porosity and excellent alloying elements, and the micro distribution of stainless steel super large freezer may be uneven. Therefore, the heat treatment process of P/M parts may be different. After decades of exploration and practice, the effect of porosity on the heat treatment performance of iron-based powder metallurgy parts has been clearly understood. The summary is as follows

1 effect of porosity on integral quenching of iron-based powder metallurgy parts

in order to increase strength, hardness and wear resistance, most iron-based powder metallurgy parts need to be integrally quenched, i.e. quenched and tempered. For iron-based P/M parts requiring overall quenching, the combined carbon content shall be ≥ 0.3% (mass fraction) and shall be in austenitic state above the A3 temperature in Figure 1

Figure 1 heat treatment phase diagram of carbon steel

the overall quenching of iron-based powder metallurgy parts consists of the following three processes:

austenitizing. In a protective atmosphere with a carbon potential equivalent to the combined carbon content, heat the parts to a temperature higher than A3, usually 850 ℃, and hold them for a certain period of time, depending on the shape and size of the parts. For example, austenitize it for 30min

quenching. Quench the parts in oil or water from austenitizing temperature or slightly lower temperature but still higher than A3 temperature to convert austenite into hard and brittle martensite or bainite. For iron-based P/M parts, it is better to quench them in warm oil (50 ℃), because P/M parts have porosity, and parts may crack when quenching and cooling speed is too fast. In addition, when salt water is used for quenching, the salt water remaining in the pores after quenching will cause serious corrosion of parts

tempering. According to gb/t "sintered metal materials - Specifications", iron-based powder metallurgy parts are usually tempered at 180 ℃ (260 ℃ for sintered nickel steel), and the tempering time is usually 1H per 25.4mm according to the section thickness of the parts. The purpose is to eliminate the internal stress generated when austenite is transformed into martensite and bainite. Tempering can reduce the brittleness of martensite and bainite and improve the toughness of parts

1.1 effect of porosity on Hardenability of powder metallurgy Fe-C materials

hardenability is defined as the ability of a material sample to transform from austenite to martensite at a given depth during rapid cooling. Hardenability is usually measured by the top quenching method. In order to determine the hardenability of sintered carbon steel, the mixed powder of water atomized iron powder and 0.9% (mass fraction) graphite powder was prepared by pressing sintering Φ 80mm × The billet with a height of 30mm and a density of 6.0 ~ 7.1g/cm3 [combined carbon 0.8% (mass fraction)]. Then, the billet is cut into the top quenched sample, which is austenitized at 870 ℃ in neutral atmosphere for 30min and then water quenched. The apparent hardness HRA shall be measured every 2.5mm from the quenched end. At same time, it is compared with the top quenched sample cut from c-1080 forged steel. The test results are shown in Figure 2

Fig. 2 Comparison of top quenching hardenability of sintered carbon steel and conventional c-1080 carbon steel [2, 3]

it can be seen from Fig. 2 that the density (i.e. porosity) of material samples has some effects on hardenability. First, porosity reduces the thermal conductivity of the material because the pores are filled with air, and the thermal conductivity of air is smaller than that of steel. In addition, the hardness indentation is related to the porosity in the material matrix, which also affects the measured hardness value. Figure 2 also shows that the hardenability of 162 interior wall coating increases linearly with the increase of sintered steel material density. Therefore, when designing a powder metallurgy carbon steel part with a given material density, figure 2 is useful for selecting a suitable material composition that enables the cross-section of the part to be fully transformed into martensite

1.2 hardenability standard for iron-based powder metallurgy materials

when designing iron-based powder metallurgy parts, in order to make the cross-section of powder gold parts change into martensite through quenching tempering, appropriate materials must be selected according to the hardenability of materials

sintered steel test bar is made of atomized iron powder mixed with 0.90% graphite, pressed and sintered, and the density is 6.0 ~ 7.1g/cm3

austenitizing at 870 ℃ in neutral atmosphere for 30min and then water quenching. Measure a point every 2.5mm from the quenching end.

for iron-based powder metallurgy materials, in addition to the above porosity, alloying elements added to the materials, such as copper, nickel, molybdenum, etc., also affect the hardenability of the materials. The American Federation of metal powder industries (MPIF) first published the hardenability standard for iron-based powder metallurgy materials in 1997. See Table 1. This standard is based on astma255 test method for top quenching

the relationship curve between hardness and depth is made with HRA hardness scale. The end quenching depth refers to the depth from the quenching end to the point where the hardness value is lower than 65hra on the sample (take a hardness reading every 1.6mm (1/16 inch). If the hardness of the sample surface does not reach 65hra, the end quenching depth (depth J) is listed as

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