Quenching Polish Quenching Heat Treatment Furnace

With the development of morden machine manufacturing industry, greater demands are being placed on the properties of metal materials. Current techonologies for metal surface reinforcement and corrosion resistance are difficult to meet these requirements.
On the other hand, for the strict restrictions to environmental protection, especially in Twentieth Century when special attentions are paid to industrial protection, many obsolete technologies are eliminated which pollute the environment a lot.
Under such circumstances, it is imperative to develop new and pollution-free technology for metal surface reinforcement.
QPQ techonology is an updated surface modification and strengthening technology for metals. The whole name is “Quench Polish Quench”. From the perspective of professional technology, this statement is not accurate enough, but has been used so far in the world.
The above “quenching” is not the general concept we know but a compound treatment for metal in two different low temperature molten salt bath. Multiple elements infiltrate the metal surface simultaneously to form composite layer of oxide film, compound layer and diffusion layer, so that the metal surface is strengthened. QPQ has something in common with conventional gas and vacuum ion nitriding, but QPQ permeates more elements and adopts different craft method. Meanwhile, it is also time-saving and nuisanceless.

Basically, QPQ technology is whole process of nitrocarburization in salt bath at low temperature, polishing and oxidation. QPQ technology in China is also called QPQ salt bath composite technology.
For technique, it is the composite of nitridation salt bath and oxidation, achieving the compound of nitriding and oxidation. For permeation layer, it is the composite of nitrides and oxides. For performance, it mainly compounds abrasion performance and corrosion resistance. For techonology, it is the combination of heat treatment technology and corrosion resistance technology.

Evolution Process of QPQ Technology

1.Gas Nitriding Method
In a nitrogen decomposition atmosphere, Nitrogen atoms produced by ammonia decomposition are infiltrated into the metal surface to produce nitride hardening layer. It must use special nitrided steel containing Cr, MO, Al, etc. Processing temperature is 510-520 ℃, and the holding time is 70 hours or more.
The drawback is that it must use special nitrided steel with special alloy elements : the treatment time is long: the surface of the compound (Fe2N) layer is too brittle and usually must be worn off.
The advantage is that it is of high hardness and deep permeation.

2.Cyanate Nitriding Method
Its raw material is highly toxic cyanide and sodium cyanide and compounds, processing temperature for 520- 560℃. The processing time will be different in accordance with different varieties, specifications, materials, generally 5-30 minutes.
The new prepared salt can only be used after a long time of aging. The purpose of aging is to make the cyanide salt absorb the oxygen from the air under the molten state, and then become cyanatewhich is the effective component of nitriding. Due to the natural imitations, cyanate content will be not too high (mass fraction<10%).
This method expands the nitriding technology from special nitriding steel to high speed steel, but it is still not applicable to other kinds of steel. It can only be used for corrosion-resistant nitriding, and the aging time is too long (several days).

3.Active Cyanide Salt Nitriding Method
Because of long aging time of cyanide nitriding salt bath, low content of cyanate root and the limited applicable type of steel, a new method of nitriding salt bath with compressed air in cyanide salt bath is developed, which greatly shortens the aging process and increases the cyanate content in the salt bath (16%, 28%).
Fe3N (including partial Fe4N) is generally formed on the surface of the workpiece. This nitride is tough, not as brittle as the Fe2N formed by nitriding steel gas after nitriding, thus called soft nitriding.
Advantages: free from restriction of steel, short process cycle, good wear resistance and fatigue resistance.
Disadvantages: cyanide is the raw material, which is highly poisonous and will cause environmental pollution.

4.Gas Nitrocarburization
In order to retain the advantages of reactive cyanide nitriding and overcome the disadvantages of highly toxic and public hazards, the gas method is adopted to simulate it instead of using highly toxic cyanide salts as raw materials. Raw material: to put urea solid raw materials into the furnace for splitting; to drip liquid raw materials such as formamide, triethanolamine into the furnace for decomposition; The gas raw material atmosphere is added with ammonia gas directly into the furnace.
Although cyanide salts are not used as raw materials, the gas is still highly toxic hydrogen cyanide (HCN).
In addition to the pollution problems, the process instability and quality control of the gas nitrocarburization all are the main problems in China.
5.Pollution-free Salt Bath Compound Treatment Method
The new composite technology developed in the mid-1970s solved the problem of pollution thoroughly. The new process uses organic and inorganic materials first to produce nitridation (TFI) consisting of hypercyanate and carbonate. Meanwhile, an organic substance called Men was used to react with the carbonate in the aging nitride bath to regenerate cyanate (CNO-).
Compared with the original method, the new material contains no cyanide at all, and the amount of salt added and the ingredients of salt bath are not cyanide. Although the raw material of the new method does not contain cyanide, cyanide root decomposition after the reaction of salt bath will produce cyanide root (CN-), accounting for 1% ~ 3% of the total mass fraction of the salt bath. Therefore, if the workpiece is cleaned directly after the nitride bath, the cleaning water cannot be directly discharged because of cyanide.
To this end, an oxidized salt has been developed specifically to break down the cyanide roots (CN-) that the workpieces take out of nitride baths. This oxidized salt bath is called the ABI in Germany and the KQ500 in the United States.
The nitride bath, which does not require cyanide to be used as raw material, together with the oxidation bath which can decompose the cyanide root, constitutes a pollution-free compound treatment technology.
The content of cyanate in nitride bath has been greatly increased, with the mass fraction of cyanate being 32% and 38%. As the nitrogen potential increases, the penetration rate of nitrogen increases, reducing the production cycle Period 30%, 35%.
At the same time, the composite processing method also generates an oxide layer on the surface of the workpiece, which is not available by other methods. This layer of oxide film greatly strengthens the workpiece’s corrosion resistance and wear resistance as well as beautifies the appearance.

Colin company in Detroit developed the new QPQ technology based on the combined treatment of pollution-free salt bath.
The process of QPQ is as follows:

It is after carrying on the salt bath compound processing complete process, add a polishing process again, after polishing do again oxidize processing again. Collectively known as QPQ new technology.
Add polish and reoxidation process to QPQ technology.
Objective: 1) remove the porous layer outside the nitride layer, and 2) replenish the oxygen content on the surface of the workpiece: strengthening corrosion resistance and wear resistance as well as beautifies the appearance of metals.

Permeability performance
As QPQ infiltrates N, C, O and other elements into the metal surface and forms a compound layer and diffusion layer on the metal surface, it greatly strengthens the wear resistance, corrosion resistance and fatigue resistance of the metal surface, which cannot be compared with single heat treatment or single corrosion resistance technology.
Because general heat treatment and surface hardening techniques can only improve wear resistance, not corrosion resistance. Most of the corrosion resistance technologies do not have high wear resistance, but QPQ technology owns both.

Abrasion Resistance
The compound layer formed on the metal surface after QPQ treatment is mainly Fe3N, which is a compound with a mass fraction of more than 6% of nitrogen. It is substantially different from the carbon supersaturated solid solution (hardened martensite) formed by ordinary heat treatment. Therefore, the two can not only compare the wear resistance with the hardness.

1) abrasive wear
When the hardness of the two metal workpiece varies greatly, it is easy to produce abrasive wear in micro cutting mode. For this type of wear, if the workpiece does not bear a large impact load, the higher the hardness of the material, the better its wear resistance.
The hardness of the metal surface after QPQ treatment is greatly improved than that of the heart, so it can greatly improve the wear resistance of the surface.
2) Fatigue Wear
As the convex part of the metal squeezes into the workpiece on the opposite side and moves forward, a bulge is formed in front of it, so the metal produces elastic deformation and the bump is removed, the surface restored. If this kind of loading and unloading is repeated, the metal surface will be fatigued due to excessive deformation times, so the metal surface will be stripped, which is fatigue wear.
The reason why QPQ treatment can improve the fatigue strength of materials is that QPQ improves the hardness of diffusion layer and maintains its high toughness. That is to say, the improvement of fatigue strength mainly lies in the effect of diffusion layer.
3)Adhesive wear
Under some friction conditions, the surface of the two matched metals will be glued due to the relatively high speed movement of the matched metal or high temperature caused by other reasons.
The function of QPQ treatment is to form a good protective film on the surface of the workpiece after the base material of the part is selected, which can greatly reduce the binding force at the cementation and reduce the occurrence of adhesive wear.
3)Corrosion Resistance
High corrosion resistance is another feature of QPQ technology. Its corrosion resistance is much better than that of chromium and nickel plating, and even better than that of some stainless steels.
QPQ treatment unit has a very high corrosion resistance is mainly dependent on the metal surface of Fe2~3N compound layer and the density of Fe3O4 oxide film. After polishing and deoxidation, it not only makes the compound layer more compact, but also make oxygen into half of the compound layer, so that metal owns great corrosion resistance in the atmosphere and salt fog, or under the condition of weak acid, strong alkali, etc.
4)Fatigue resistance
QPQ technology can significantly improve the fatigue resistance of materials.
The improvement of fatigue resistance mainly depends on the action of diffusion layer below the compound layer.
The improvement of fatigue strength of carbon steel is mainly due to the precipitation of Fe8N from supersaturated ferrite at room temperature after cooling knife.
These two types of precipitation can hinder the movement of dislocation and thus increase the fatigue strength.

Dimension and shape of workpiece
The minimum variation of workpiece size and shape before and after processing is a major feature of QPQ technology.
Reason: the temperature of QPQ process is lower than the phase transition temperature of steel. During the processing, the metal does not undergo tissue transformation, so no tissue stress is generated. Therefore, it is much less deformation than conventional quenching, high frequency induction quenching and carburizing quenching, where tissue transformation occurs.

Application of QPQ Technology
QPQ technology is applicable to all kinds of wear resistant parts, corrosion resistant parts, fatigue resistant parts or some parts combined by required wear resistant and fatigue resistant, or wear resistant and corrosion resistant .
This technology has been widely used in automobile, motorcycle, locomotive, construction machinery, agricultural machinery, light chemical machinery, textile machinery, instrument and meter, machine tool, gear, machinery, tools, mold and other industries.

Typical parts of typical industries are as follows:
Auto parts: crankshaft, camshaft, valve, the ball head pin, piston, gas spring, torsion plate, the slippery course of seat slider, the modulus of flywheel brake control system, bumper, helical gear, internal gear reducer, a vertical driving gear, door lock, shaft sleeve, windshield rocker arm, suspension strut piston, motor cooling fan, speakers, prevent bask in the actuator.
Mechanical parts of textile machine: various wear-resisting, corrosion-resistant parts of winder, nylon elastic wire machine, various wear-resisting, corrosion-resistant parts of card and roller, stator, steel ring and other textile parts.
Machine parts: guide rail, lead screw, shaft, friction plate, gear and machine tool electrical core.
Gears: some automobile gears, motorcycle gears, all kinds of small modulus gears, motor gears, instrument gears, all kinds of large modulus low-speed heavy-duty gears, various specifications of worm and so on.
Various molds: die-casting mold, extrusion mold, stretch film, forging mold, plastic mold, glass mold, ceramic mold, ceramic mold, etc.
Various tools: various high-speed steel or Cr12M. V steel cutting tools, including drill, milling cutter, reamer, tap, hob, gear cutter, shaving cutter and broach, thread wheel, thread rolling plate, all kinds of machine clamping tools and various kinds of CNC cutter body.

Current usage of QPQ at home and abroad
The technology has been widely used in more than 40 countries, including Germany, the United States, Britain and Japan
Many domestic manufacturers have also adopted this technology, including crankshaft, valve, camshaft, piston ring, air spring piston rod, mould, cutter, etc.
Currently, the shiyan automobile industry has been developing rapidly and has become the largest production base for commercial vehicles. However, the QPQ technology has not been applied yet, so it is urgently needed to carry out technological transformation and upgrading.

Comparison and Analysis of above treating techniques
Gas nitriding
Nitrogen atoms produced by ammonia decomposition infiltrate the metal surface to produce nitride hardened layers.
Disadvantages:
1) special nitrided steel containing special alloy elements must be used;
2) long processing time (temperature keeping time is more than 70 hours).
Ion nitriding
Using the principle of glow discharge, nitrogen and hydrogen positive ions bombard the surface of the workpiece at high speed under the action of strong electric field in the plasma. At the same time, nitrogen infiltrates into the surface of the workpiece due to the action of adsorption and diffusion.
Disadvantages:
1)ion nitrification time still needs 30 or 40 hours; Complex equipment and large investment;
2)it is difficult to accurately measure part temperature;
3)it is difficult to ensure the same workpiece temperature for the large furnace and various parts.
Carbonitriding
The chemical heat treatment process of infiltrating carbon and nitrogen into the surface of the part at the same time is mainly carburizing. The co-carburizing temperature is lower than carburizing and higher than nitriding (800, 880 ℃).
Advantages:
1)the temperature is lower than carburizing temperature, the grain will not grow, suitable for direct quenching, and the rate of co-carburization is fas;
2)Quenching deformation and cracking are reduced;
3)High wear resistance and fatigue strength, low brittleness
Disavantages:
1)solid carbonitriding has poor quality and long period;
2)liquid carbonitriding is carried out in salt bath containing cyanide, which is highly toxic;
3)gas carbonitriding is widely used at present.
Carburizing and Quenching
Working principle of carburizing: in order to increase the carbon content and a certain carbon concentration gradient on the surface of the workpiece. Place the workpiece into the carburizing medium for heating and heat preservation, so that carbon atoms can penetrate into the surface of the chemical heat treatment process.
Performance after carburizing and quenching: obtain high surface hardness and high contact fatigue.
Classification: solid carburizing, liquid carburizing, gas carburizing
Disadvantages:
1)can only improve surface performace;
2)need two process of carburizing and quenching;
3)when the cementation layer is in the state of compressive stress after quenching, the residual compressive stress of the surface will be reduced by tempering, and the bending fatigue strength of the part will be reduced.
Galvanization
Working principle of galvanization:
Put the workpiece in a galvanized metal bath containing galvanized liquid. The workpiece is cathode. The galvanizing bath and galvanizing liquid are anode. After passivating, dyeing and coating, The workpiece can significantly improve the workpiece protection and decoration, and the direct cost is lower.
Disadvantages:
1)good corrosion resistance in dry air, but tight zinc carbonate film is produced on the surface of zinc in humid air;
2)the galvanized solution is divided into cyanide plating solution and non-hydrogenated plating solution. Cyanide plating solution is widely used in production.
Chrome-plating
Chromium-plating is based on the same principle as galvanizing, except that the chromium-plating bath is an insoluble anode made of lead, lead and antimony alloy. Chromium plating can make parts smooth and flat, rustless, and hard (up to HRc65) resistant to high temperature (500℃) corrosion resistance, acid resistance and wear resistance.

Disadvantages:
1)the surface roughness of the workpiece before chromium plating is high, and the precision affects the surface quality of chromium plating;
2)during chromium plating, the volatile bands of hydrogen and oxygen release chromium ions, which are harmful to human health and pollute the environment with waste plating solution;
3)the process time is related to the thickness of the coating layer. After plating, the workpiece needs to be processed and the external dimension of the workpiece should be increased, which is not suitable for the workpiece with complex external dimensions;
4)fatigue and poor contact strength. Under local stress, the surface chrome layer is easy to peel off.
Main Characteristics of QPQ Technology
The workpiece processed by QPQ
1) wear resistance, corrosion resistance, fatigue resistance, bite resistance and abrasion resistance
2) short process time (6h), low temperature (< 600℃), small deformation, no need for subsequent finishing
3) pollution-free, no harms to the health of operators, pollution-free wastes.
What original technologies can be replaced by QPQ technology?
In order to improve the wear resistance of workpiece surface, this technology can replace the heat treatment and surface strengthening technologies which adopt massive carburizing quenching, high-frequency induction quenching and ion nitriding.
Additionally, for improve the corrosion resistance of workpiece surface, the surface protection technologies such as blackening, chromium plating, hard chromium plating and nickel plating can also be replaced.

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