Thu, 24 May 2018 08:23:53 +0000
Cause of oxidation during sintering of powder metallurgy parts
The oxidation and decarbonization of iron base parts in powder metallurgy often occur at the same time. It is often seen that the surface of sintered iron base parts is bluish, or the hardness value of products after sintering is lower than the standard value, and the pearlite volume is less in the metallographic examination. As a result, the appearance and performance of the product are not qualified, which affects the quality of the product. For oxidation and decarburization, many manufacturers don’t know how to solve them, some manufacturers try to increased quantity of graphite in mixing, hope for the increase of graphite can offset the oxidation decarburization, at the same time have the effect of carbon oxygen at high temperatures. However, the increase of pores caused by excessive carbon addition reduces the strength of the product and pollutes the iron matrix. Obviously, the side effects of excessive increase of graphite should not be ignored. The process of sintering iron base parts in pusher furnace and mesh belt furnace is analyzed in depth, and the root cause of the above phenomenon is found out.
1. The iron-based powder metallurgy parts sintering process of oxidation and decarburization phenomenon on the quality of the products the oxidation of ferrous parts sintering process is divided into three types: the first is the early stage of the sintering oxidation, the second for sintering late oxidation, and the third for hybrid oxidation. Pre-sintering oxidation refers to the oxidation from room temperature to dewaxing. The oxidation of sintering after sintering is the oxidation of sintered billet in the cooling area after sintering. Mixed oxide product which is affected by the previous oxidation before dewaxing, in cooling phase is under the influence of the late oxidation, namely in the whole sintering process under the influence of early and late oxide oxidation at the same time, is one of the most serious oxidation phenomenon.
1, 1 compact from room temperature to the front of the dewaxing oxidation oxidation (early) this kind of oxidation is usually can’t see the blue phenomenon, after the product surface is rough and no metallic luster, hardness is usually lower than standard values, pearlite in the microstructure, less serious decarburization phenomenon. Microstructure and properties of normal and decarburization products material composition compact density/g. cm – 3 normal product hardness HB phase composition decarburization product hardness HB / % pearlite ferrite pearlite ferrite phase composition / % more than note: decarburization is most serious, can not see the existence of pearlite in metallographic.
1, 2 sintering late this oxidation oxidation phenomenon for the push rod type sintering furnace mesh belt sintering furnace and sintering are exist, oxidation at low temperature, short time, just appear blue phenomenon on the surface of the product, its oxidation is, on the surface of the decarburization is also in the surface layer, product performance is hardly to reduce, this type of oxidation mainly influence the appearance quality of the products. When the oxidation temperature is high or the oxidation time is too long, the product will also have serious decarburization, desquamation and seriously affect the performance of the product, resulting in the product being scrapped.
1, 3 mixed oxide product after sintering, the late before oxidation at the same time, the introduction of products not only low hardness, appearance of blue or yellow, the worst will happen when the phenomenon of skin desquamation, most can only be scrapped after the oxidation products. In short, whatever the oxidation, chemical reactions in oxygen and iron at the same time, will and carbon oxygen decarburization reaction and make the iron base parts, due to the nonuniformity decarburization, makes the performance difference from different parts of the same product become bigger and affect the quality of the product.
2. When different sintering equipment of powder metallurgy iron base parts of the oxidation of 2, 1 in the push rod type sintering furnace sintering in sintered iron base parts in pusher-type sintering furnace, late early oxidation, oxidation and hybrid oxidation are likely. The push-rod type sintering furnace consists of feed push rod, feed end, furnace body and cooling water jacket. Temperature setting for each temperature: 600 ℃, 960 ℃, 1030 ℃, 1080-1030 ℃; The feeding terminal is usually arranged with 3 boxes of products, 16 boxes of products for heating area and 8 boxes for cooling end. Practice shows that the first box of products (1, 2, 3… ) exit after 5min after entering the furnace. A large amount of water wets the whole box of products. The second box has less water than the first box, and the surface of the blank is vaporized and the color is straw yellow. There was no water in the third box, and the blank was dried to a dark yellow. In observation of cooling water in the oxidation of ferrous products phenomenon is found that when the water cooling tank wall adherent fine droplets, products after the intermittent blue phenomenon, every 4 sintering box blue occur will have a box products. Sometimes there will be a continuous blue phenomenon. Push-rod furnace, of course, there is a before, the oxidation of late phenomenon, is inevitable phenomenon that there are two appear together, often can see sintering product not only appearance, performance is unqualified, this is the so-called hybrid oxidation.
2. 2. The structure of sintering mesh belt furnace in the net belt type sintering furnace is different from that of the putter type furnace, which is composed of the network belt transmission mechanism, the dewaxing section furnace body, the sintering section furnace body and the cooling water jacket. RBO dewaxing device is used to replace the dewaxing section furnace body with rapid dewaxing agent. In fact, in the use of mesh belt furnace sintered iron base products, found no similar push rod type sintering furnace as condensed water dipping compact, therefore there is no the oxidation of the early stage of the sintering phenomenon. However, the blue phenomenon of product is very common, that is, the phenomenon of late sintering is frequent. Naturally, because there is no pre-oxidation, mixed oxidation will not occur.
3. The influence of different protective atmosphere on product decarbonization. It is difficult for furnace gas to control the dew point of atmosphere and the carbon potential of sintering process, which makes it difficult to obtain the ideal performance of sintered products. Hydrogen is a reducing gas, it has a strong reduction ability, ammonia decomposition and nitrogen base atmosphere are different in the content of hydrogen, and thus determine the atmosphere reduction ability. The decomposition ammonia contains 75%H2 and 25%N2, while the nitrogen atmosphere contains only 10% to 15% hydrogen. Obviously, the reduction capacity of ammonia decomposition gas is far greater than that of nitride. Because of its different properties, it has different effects on the sintering process of iron base parts. H2 has decarburization, and the dry hydrogen gas is carried away with the carbonation of the steel in a certain condition. Production practice, the decomposition of NH3 under protective atmosphere sintering iron base products, mixing with graphite quantity is 1, 1% – 1, 1% (mass fraction), sintering under nitrogen based atmosphere are simply add 0, 6% and 0, 6% of graphite (mass fraction). Obviously, decomposing ammonia is much stronger than the nitrogen base atmosphere.