Decarburization of High Speed Steel by Salt Bath Quenching: Analysis and Prevention

Due to various reasons, decarburization of workpieces during salt bath heating and quenching is unavoidable.

If the decarburization layer is relatively thin, it will be fine if the subsequent grinding can be completely removed;

If the decarburization layer cannot be worn off, the quality of the product will be seriously affected.

1. Effect of decarbonization on product quality

Decarburization refers to the phenomenon that the carbon content on the surface of steel decreases during heating, or the combustion of carbon on the surface of steel, i.e. C + O2 → CO2.

The essence of decarburization is that the carbon on the surface of steel reacts with oxygen, hydrogen, etc. at high temperature, resulting in changes in the surface state, thus affecting the quality of heat treatment.

It is mainly shown in the following aspects.

(1) Early damage failure

Decarburization of quenched workpiece reduces the surface hardness of quenched workpiece and affects its service life.

For example, φ40mm×1mm screw groove milling cutter made of W18Cr4V steel, the microstructure of the tooth tip surface is white crystalline decarburization layer (microhardness 338HV) + a small amount of black troostite transition zone (microhardness 627HV) + normal tempered martensite and a small amount of carbide (microhardness 825HV), as shown in Fig. 1.

The decarburization layer is distributed along the tooth profile.

It can be seen that decarburization occurs during the heat treatment.

The milling cutter is worn out early because of its low hardness.

Fig. 1 quenching and decarburization of screw groove milling cutter

(2) Cause quenching crack

Quenching decarburization will cause the carbon concentration of the inner and outer layers of the workpiece to be different, and cause the transformation of the inner and outer layers in the heat treatment phase transformation process, and the volume expansion and contraction are also different.

Therefore, large structural stress and thermal stress are generated, and microcracks are easily generated at the stress concentration of the workpiece.

(3) Reduce fatigue strength

Decarburization of workpieces has a great impact on their fatigue strength, especially for tools in service under the condition of reciprocating alternating stress, which is very sensitive to its life. The main characteristics of failure are surface peeling, fracture and other early damage.

(4) Surface corrosion

When the quenched molten salt is used at high temperature, sulfate, carbonate and water will react with the workpiece, causing oxidation and decarburization in light cases and corrosion pitting in severe cases.

In addition, when the surface of the workpiece is processed, copper sulfate is used for chemical copper plating due to process requirements.

If the surface copper plating layer is heated at high temperature before it is removed, the surface of the workpiece after quenching will also be corroded.

(5) Burn workpiece

In order to keep the salt bath clean and prevent decarburization of quenched parts, most factories add appropriate deoxidizer to the molten salt, but it is easy to burn the workpiece if the construction is improper.

Fig. 2 shows the scrapped M2 steel φ46mm×1mm screw slot milling cutter burned by the deoxidizer ferrosilicon.

The metallographic structure at the burning point is secondary ledeburite, and the black belt at the junction with the normal structure is troostite (carbon poor area), as shown in Fig. 3.

Fig. 2 appearance of burned milling cutter

Fig. 3 metallography of burned milling cutter

The most intuitive result of decarburization is that the tool life decreases or even cannot be used.

2. Causes of decarburization

The main reasons for decarburization caused by salt bath quenching are as follows:

(1) There are harmful impurities in the salt bath, such as Na2SO4, BaSO4, Na2CO3, CaCO3, BaCO3, etc., which promote decarbonization in the salt bath.

(2) The more water in the air dissolves into the salt bath, the easier decarbonization will be caused.

(3) There is rust on the workpiece or quenching fixture.

During the heating process of the salt bath, these oxide scales increase the oxide content in the salt bath and correspondingly increase the oxygen content in the salt bath, thus promoting the decarburization of the workpiece.

(4) The quenching hook is stuck with nitrate, or the chloride salt is mixed with nitrate, which will also increase the oxygen content in the salt bath.

(5) Insufficient deoxidation in salt bath or incomplete slag removal.

(6) The salt bath was not replaced in time due to aging.

3. Determination of decarbonization in salt bath

There are many methods to judge whether the salt bath decarburizes.

The simplest method is to use a file, but it must have rich on-the-spot experience.

There are also steel foil test method, chemical analysis method, hardness method and micro structure determination method.

The following is a brief introduction.

(1) File judgment

For example, for decarburization by salt bath quenching, because the hardness of its part is lower than that of the core, a V-shaped groove is filed with a file with a hardness ≥ 66HRC, and the decarburization layer is determined according to the groove depth.

The hardness and decarburization layer of the high-speed steel tool are determined by the self-made file made by some tool manufacturers.

The file material is 4341 high-speed steel, and the finished file is subject to surface strengthening such as QPQ.

(2) Macro corrosion judgment

After the sample or test piece quenched by salt bath is heated at 700 ℃ and immersed in 20% hydrochloric acid for several minutes, it is cleaned and observed.

Because the decarburized part and non decarburized part have different corrosion resistance, the decarburized part is white.

The depth of decarburized layer can be determined by observing with a magnifying glass.

(3) Hardness determination

Measure the hardness distribution of the cross section of the sample subjected to salt bath heat treatment.

If the hardness of the surface is different from that of the core, the decarburized layer is determined where the hardness is the same as that of the core.

Some people also think that the hardness boundary for judging whether the high-speed steel is decarburized after heat treatment is 823HV0-1, and the thickness from this point to the surface is the depth of the decarburized layer.

(4) Steel foil test

Steel foil (mass fraction 1.0% C steel strip: 0.5mm thick × 30mm wide × 150 mm long) is placed in a salt bath and heated to the specified time under the conditions in Table 1, then quenched into 10-30 ℃ flowing tap water for cooling, and broken by hand to see the actual breaking, and determine the decarburization and deterioration of the salt bath according to the judgment criteria in Table 2.

Table 1 heating time of steel foil in salt bath

Salt bath heating temperature / ℃8009001000110012001300
Heating time / min201510532

Table 2 determination criteria for steel foil

CharacterBreaking conditionCarbon content of steel foil (%)Decarburization rate of steel foil (%)Is the salt bath suitable
1Brittle when broken>0.6030~40
2Elastic when broken0.40~0.5050~60
3Only after twists and turns can it break0.20~0.3070~80×
4The twists and turns are endless<0.2080~90×

(5) Metallographic determination

As shown in Fig. 4, the grain size on the decarburized surface layer of quenched high-speed steel workpiece is relatively coarse.

Fig. 4 metallography of decarburized milling cutter

4. Measures to prevent decarbonization

(1) The rust attached to the electrode handle and fixture must be removed before quenching.

(2) The accumulated slag formed by the oxide in the molten salt or the floating objects on the liquid surface shall be removed timely.

(3) Whether the workpieces are dry or not, they must be baked and dried.

(4) The salt attached to the workpiece or fixture or the scattered salt cannot be brought into the salt bath due to the high degree of oxidation, and must be prevented and removed.

(5) The molten salt is prevented from reacting with the atmosphere to improve the alkalinity.

In order to prevent and reduce the disease, the following measures can be taken:

① The salt bath temperature shall be as low as possible;

② The surface area of salt bath shall be as small as possible;

③ The furnace cover shall be covered if it is not quenched;

④ The surface of the bath liquid is covered with a circulating gas flow of nitrogen or an inert gas.

(6) When the furnace is shut down for a long time, the salt solution shall be taken out, dried, broken into small pieces, and then placed in a dry place for storage.

If too many black mud or inclusions are found, they shall be scrapped and no longer used.

(7) Fully deoxidize and fully and thoroughly remove slag.

(8) Non deoxygenated long-acting salt or 5% MgF2 high-temperature salt bath (95% BaCl2) was used.

(9) Salt purchased from regular channels shall be sampled and tested before entering the factory and warehousing, and can be used only after it is qualified.

(10) Do a good job in site management and eliminate decarbonization factors in the bud.

5. Conclusion

Salt bath furnace is still the main heating equipment for heat treatment of high-speed steel at present.

Although the vacuum furnace develops rapidly, it can not completely replace the salt bath furnace.

In a long period of time, the advantages of the two are complementary and coexist for a long time, but the salt bath will eventually withdraw from the historical stage.

At present, it is very important to pay attention to the quality of salt bath quenching, especially to prevent decarburization.

Only by paying close attention to every link can we ensure less decarburization and no decarburization, thus ensuring the high service life of tools.

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