Notes on carburizing heat treatment

Common problems of carburizing heat treatment process:

(1) high carbon concentration

Causes and hazards: if carburizing heating, temperature is too high or solid carburizing with new carburizing agent, or with strong carburizing agent will cause carburizing over the phenomenon of high. With the high carbon concentration, massive carbides or network carbides appear on the surface of the workpiece. Because this kind of hard brittle tissue produces, make the toughness of carburized layer drops sharply. And the formation of high carbon martensite during quenching, grinding cracks easily appear.

2. Methods of prevention

Can not be heated rapidly, need to use appropriate heating temperature, do not make the grain of steel grow up as well. If the grain is coarse when carburizing, it should be normalized or quenched twice after carburizing to refine the grain.

(2) strictly control the furnace temperature average, can not fluctuate too much, in the reflection furnace when the solid carburizing need to pay special attention.

(3) solid carburizing, carburizing agent to new, old ratio of use. Osmotic agent is best to use 4-7% BaCO3, not Na2CO3 as osmotic agent.

(2) carbon concentration is extremely low

Causes and hazards: temperature fluctuation is very large or carburizing agent too little will cause insufficient surface carbon concentration. The optimal carbon concentration is 0.9 -- 1.0%, lower than 0.8%C, the parts are easily worn.

2. Methods of prevention:

(1) carburizing temperature is generally used 920-940 ℃, carburizing temperature through low will cause carbon concentration through low, and prolong the carburizing time; High carburizing temperature will lead to coarse grain.

Osmotic agent (BaCO3) dosage should not be less than 4%.

(3) locally poor carbon on the surface after carburizing

Causes and hazards: solid carburizing, charcoal particles are too large or with stones and other impurities, or carburizing agent and charcoal mix unevenly, or workpiece contact will cause local carbon or poor carbon. The dirt on the workpiece surface can also cause poor carbon.

2. Methods of prevention

Solid carburizing agent must be prepared in proportion, stirring average.

Pay attention to the furnace workpiece do not have contact. Solid carburizing should be ramming carburizing agent, do not make carburizing collapse and make the workpiece contact.

But remove the dirt on the surface.

(4) carburizing concentration intensifies the transition

Causes and hazards: carburizing concentration nonstop transition is the surface and central carbon concentration change intensification, not from high to low average transition, but nonstop transition. The reason for this defect is that carburizing agent action is very strong (such as the new preparation of charcoal, the old carburizing agent added very little), at the same time, there are Cr, Mn, Mo and other alloy elements in the steel is to promote the formation of strong carbides, and resulting in high surface concentration, low central concentration, and no transition layer. This defect causes internal stress proportional to the inside and outside, resulting in cracks or spalling during quenching or grinding.

The method of prevention: carburizing agent new and old by delimited proportion system, make carburizing moderate. It is better to use BaCO3 as impermeable agent, because Na2CO3 is more rapid.

(5) temper and crack occur during grinding

Causes: the surface of the carburized layer after grinding causes softening phenomenon, known as the tempering mill. This is because the grinding feed is too fast, the grinding wheel hardness and particle size or speed selection is not appropriate, or the grinding process cooling is not sufficient, are easy to produce such defects. This is due to the surface softening due to the heat during grinding. Tempering defects in grinding reduce the wear resistance of parts.

Hexagonal cracks appear on the surface. This is caused by excessive grinding on the surface of the hard grinding wheel and fever. It is also related to insufficient tempering of heat treatment and excessive residual internal stress. After etching with acid, the general defect is dark color, can be distinguished from the defect. This is the heat generated during grinding tempering. The cause of the transformation of a horse body into troosomal tissue. In fact, the cracks are visible to the naked eye after grinding.

2. Methods of prevention:

(1) after quenching, the internal stress must be eliminated by sufficient tempering or multiple tempering.

The use of 40~60 particle size of soft or medium alumina grinding wheel, grinding feed is not large.

When grinding, first open the coolant, and pay attention to the process of sufficient cooling

(6) characteristics of carbon steel

(1) the carbon content of the carburizing steel is generally in the range of 0.15--0.25%, for heavy carburizing, can be advanced to 0.25--0.30% carbon carburizing steel, the most used are 15 and 20 steel, after carburizing and heat treatment surface hardness up to 56--62HRC. But because hardenability is low, only for the core strength requirements, small force, bear wear small parts, such as sleeve, chain and so on.

(2) the role of alloying element in carburizing steel hardenability is improvement, refine the grain, strengthening, solid solution, influence the carbon content, the permeability layer thickness and permeability layer organization. Usually join in carburizing steel alloy elements are manganese, chromium, nickel, molybdenum, tungsten, vanadium, boron, and so on. Low alloy carburizing steel such as cr, 20 cr2mnvb, 20 mn2tib etc, its permeability than carbon intensity of exudative and carburizing steel is high, can be used to make important carburized parts of general machinery, such as automobile, tractor gear, piston pins, etc. In the alloy carburizing steel such as 20 cr2ni4, 18 cr2n4w, 15 si3 MoWV, because of its high hardenability and high strength and toughness, is mainly used for manufacturing parts with large cross section, heavy load and complicated force, such as gears and shafts for airborne transmission.

Solid carburizing; Liquid carburizing; Gas carburizing - carburizing temperature is 900-950 c, surface layer w (carbon) 0.8-1.2%, deep layer 0.5 -- 2.0 mm.

After carburizing heat treatment - carburized workpieces in fact should be regarded as a kind of surface and central content difference code composites. Carburizing can only change the carbon content on the surface of the workpiece, and the ultimate strengthening of its surface and the core is necessary through appropriate heat treatment can be achieved. After carburizing and quenching and low temperature tempering is artifacts that are required. The purpose of quenching is made on the surface to form high carbon martensite or high carbon martensite and fine granular carbides organization. Low temperature tempering temperature is 150-200 c.

(7) matters needing attention in carburizing heat treatment

(1) normalizing pretreatment before carburizing -- the purpose is to improve the original structure of the material, reduce banding, eliminate weihlisteria structure, make the surface roughness become thinner, and eliminate the unfair state of the flow line of the material. Air cooled with 860--980C, 179--217HBS.

(2) the hardness of the workpiece to be machined after carburizing shall not be higher than 30HRC.

(3) for carburized and quenched parts with thin-wall grooves, the thin-wall grooves shall not be processed before carburizing.

(4) it is not allowed to use galvanizing method to prevent carburizing.

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