Sintered powder metallurgy particles are easy to be automatically bonded or agglomerated, especially very fine powders, which can gradually agglomerate even after a long time at room temperature. Under high temperature, the powder blank will bond between particles, which is often called sintering phenomenon. After the powder is sintered, the strength of the sintered body will increase. First, the connection strength between particles is improved, that is, the gravity between atoms on the connection surface is increased. In powder or powder compacts, the number of atoms on the contact surface between particles that can reach the atomic gravity range is limited. However, at high temperature, the amplitude of atomic vibration will increase and diffuse. More atoms on the contact surface will enter the range of atomic force, forming a bonding surface. With the increase of the bonding surface, the strength of the sintered body will also increase.

The increase of sinter strength is also reflected in the decrease of pore volume and total number and the change of pore shape, which is represented by spherical particles. The size and number of holes are also changing. In other words, the number of holes decreases, the average hole size increases, and small holes are easier to shrink and disappear than large holes.

The formation of particle bonding surface generally does not lead to the shrinkage of the sinter, so densification does not mean the beginning of the sintering process. Only the strength of the sinter is improved, which is an obvious sign of sintering. With the growth of sinter, the total pore volume decreases, the distance between particles shortens, and the densification process of sinter actually begins. Therefore, the isothermal sintering process of the powder can be divided into three stages according to time.

(1) Bonding stage - in the early stage of sintering, the original contact points or surfaces between particles will be converted into crystal bonding, and sintered wood will be formed through atomic processes such as nucleation and crystal growth. At this stage, the particles in the particles remain unchanged, the particle shape is almost unchanged, the whole sinter does not shrink, and the density increases very little, but the strength and conductivity of the sinter increase significantly due to the increase of the particle interface.

(2) Sintering growth stage - a large number of atoms move to the particle junction surface, making the sintered wood larger, the distance between particles narrower, and forming a continuous mesh. At the same time, with the growth of the grain boundary, the grain boundary will move over the vacancies, and the holes will disappear in a large number where the grain boundary is swept. Shrinkage, density and strength increase of sinter are the main characteristics at this stage.

(3) Closed hole shrinking stage - when the density of the sintered body reaches 90%, most of the holes are completely separated, the number of closed holes increases significantly, the hole shape is close to spherical, and continues to shrink. At this stage, the whole sinter can still contract slowly, but mainly due to the disappearance of small holes and the reduction of the number of holes. This stage can last for a long time, but there are still a few isolated holes that cannot be removed. The relative length of the three stages of isothermal sintering is mainly determined by the sintering temperature. Low temperature, only one stage can occur. Under production conditions, at least the second stage shall be ensured to be nearly completed. The higher the temperature is, the earlier the second or third stage occurs.

Lianyungang is equipped with four crawler type sintering furnaces, which are required to have a sintering protective atmosphere and have three temperature zones. It is divided into pre tropical zone, high temperature zone and cooling zone.

1. Pre heating belt

The set temperature here is higher than the decomposition temperature of the lubricant in the compact, so in this temperature zone, the lubricant is decomposed into gaseous products and taken out of the furnace by the sintering atmosphere.

2. Temperature band

Complete the sintering of the compact, including heating up to the sintering temperature and holding for a period of time. The state of the high temperature zone is critical to ensure the performance of the compact, because it is in the high temperature zone. The metal reacts with the sintering atmosphere and the interior of the compact solidifies. Only high enough temperature can accelerate the completion of these reactions.

3. Cooling belt

The sintered parts are gradually reduced from high temperature to room temperature through the cooling belt. No matter how the furnace is heated and how the parts are transported, the continuous sintering furnace has these three basic temperature zones. The preheating zone and high temperature sintering belt of many furnaces are separated by a fire curtain.

Before the compact enters the high temperature sintering zone, the lubricant must be burned clean, otherwise the residual decomposition products (such as zinc decomposed from zinc stearate) will pollute the furnace body and reduce the product quality. If the lubricant cannot be completely burned off in the pre heating zone, another pre sintering furnace can be used to burn off the lubricant before the compact is sintered in a very pure atmosphere or vacuum.

An unforeseen problem is power failure. Many factories are located in remote suburbs and occasionally face power failure, so sintered parts are scrapped due to too long time in the sintering furnace.