Hard turning refers to the turning of hardened steel as the final machining or finishing process to avoid the grinding technology currently in common use.

Hard Turning

Turning is one of the most basic, broadest and most important processes in the machinery industry. It directly affects production efficiency, cost, energy consumption and environmental protection.

Basic Information

Due to the development of modern science and technology, various high-strength and high-hardness engineering materials are increasingly used. Traditional turning technology is difficult or impossible to realize the processing of certain high-strength and high-hardness materials. Hard turning technology makes this possible and delivers significant benefits in production.

Hardened steel generally refers to a workpiece material having a martensite structure after quenching, high hardness, high strength, and almost no plasticity.

When the hardness of the hardened steel is >55 HRC, the strength sb is about 2100 to 2600 N/mm2.

Usually, the workpiece has been subjected to the roughing process before the heat treatment is hardened, and only the finishing is performed in the hardened state.

Fine grinding is the most commonly used processing technology for finishing, but its processing range is narrow, investment is large, production efficiency is low, and it is easy to cause environmental pollution, which has been plaguing the economical and efficient processing of hardened steel.

With the development of processing technology. Hard turning has become possible instead of grinding and has achieved significant benefits in production.

At present, the hardened steel (55-65HRC) is machined on a lathe or turning center using polycrystalline cubic boron nitride (PCBN) tools, ceramic tools or coated carbide tools.

Hard Turning Features

High processing efficiency

Hard turning has higher processing efficiency than grinding, and it consumes 1/5 of the energy of ordinary grinding.

Hard turning often uses large cutting depths and high workpiece rotation speeds, and the metal removal rate is usually 3-4 times that of grinding.

In the turning process, a variety of surface processing (such as outer circle, inner hole, slot, etc.) can be completed in one clamping, while grinding requires multiple installations.

Therefore, the auxiliary time is short and the positional accuracy between the machined surfaces is high.

Hard turning is a clean processing process

In most cases, hard turning does not require coolant.

In fact, the use of coolant can adversely affect tool life and surface quality.

Because hard turning is formed by making the material of the sheared portion soft and annealed.

If the cooling rate is too high, the effect caused by the cutting force is reduced, thereby increasing mechanical wear and shortening tool life.

At the same time, hard turning can save the coolant-related devices, reduce production costs, simplify the production system, and form chips that are clean and easy to recycle.

Low equipment investment, suitable for flexible production requirements

When the productivity is the same, the lathe investment is 1/3 to 1/2 of the grinding machine, and the cost of the auxiliary system is also low.

For small batch production, hard turning does not require special equipment, while high-volume machining of high-precision parts requires CNC machines with high rigidity, positioning accuracy and high repeatability.

The lathe itself is a flexible processing method with a wide processing range.

Turning and clamping is fast, and modern CNC lathes with multiple tool turntables or magazines make it easy to convert between two different workpieces. Hard turning is especially suitable for this type of machining.

Therefore, hard turning is better suited to flexible production requirements than grinding.

Hard turning allows parts to achieve good overall machining accuracy

Most of the heat produced in hard turning is carried away by the chips, which does not cause surface burns and cracks like grinding. It has excellent surface quality and precise roundness to ensure a high position precision between the machined surfaces.

Hard turning tool materials and the selection

Coated cemented carbide

Coated cemented carbide tools are coated with one or more layers of wear-resistant TiN, TiCN, TiAlN and Al2O3 on a toughness hardened carbide tool. The thickness of the coating is 2-18μm.

Coatings usually play the following two roles:

On the one hand, it has a much lower heat transfer coefficient than the tool base and workpiece material, which weakens the thermal action of the tool base; on the other hand, it can effectively improve the friction and adhesion of the cutting process and reduce the generation of cutting heat.

Compared with cemented carbide tools, coated carbide tools offer significant improvements in strength, hardness and wear resistance.

For the turning of workpieces with dry hardness between 45 and 55 HRC, low-cost coated carbide can achieve high-speed turning.

In recent years, some manufacturers have also greatly improved the performance of coated tools by improving the coating materials and proportions.

For example, some manufacturers in the United States and Japan use Swiss AlTiN coating materials and new coating patented technology to produce blades with hardness of 4500 to 4900 HV. When the turning temperature is as high as 1500 ° C ~ 1600 ° C, the hardness is still not reduced and does not oxidize.

Blade life is 4 times that of general coated blades.

The cost is only 30% and the adhesion is good.

It can process die steel with a hardness of 47-52HRC at a speed of 498.56m/min.

Ceramic material

Ceramic tools have high hardness (91 ~ 95HRA), high strength (bending strength of 750 ~ 1000 MPa), good wear resistance, good chemical stability, good anti-blocking performance, low friction coefficient and low price.

When used normally, it has extremely high durability and can be 2 to 5 times faster than cemented carbide. It is especially suitable for high hardness material processing, finishing and high speed machining. It can process all kinds of hardened steel and hardened cast iron with hardness of 62HRC.

Commonly used are alumina-based ceramics, silicon nitride-based ceramics, cermets, and whisker toughened ceramics.

In recent years, through a large number of research, improvement and the adoption of new manufacturing processes, the flexural strength and toughness of ceramic materials have been greatly improved.

For example, the new cermet NX2525 developed by Mitsubishi Metal Corporation of Japan and the new CT series and coated cermet blade series of cermet inserts developed by Swedish Sandvik Coromant.

The diameter of the grain structure is as small as 1 μm or less, and the flexural strength and wear resistance are much higher than those of ordinary cermets, which greatly expands the application range of ceramic materials.

The silicon nitride ceramic material tool successfully developed by Tsinghua University has also reached the international advanced level.

CBN

CBN is second only to diamond in hardness and wear resistance and has excellent high temperature hardness.

Compared with ceramic knives, its heat resistance and chemical stability are slightly poor, but impact strength and crush resistance are better.

It is widely used in the cutting of hardened steel (above 50HRC), pearlitic gray cast iron, chilled cast iron and high temperature alloy.

Compared to carbide tools, the cutting speed can be increased to a higher level.

PCBN tools with high CBN content have high hardness, good wear resistance, high compressive strength and good impact toughness.

Its shortcomings are poor thermal stability and low chemical inertness, suitable for the cutting of heat-resistant alloys, cast iron and iron-based sintered metals.

The composite PCBN tool has a low CBN particle content and uses ceramic as a binder. Its hardness is low, but it compensates for the poor thermal stability and low chemical inertness of the former material, and is suitable for the cutting of hardened steel.

In the field of cutting gray cast iron and hardened steel, ceramic tools and CBN tools can be selected at the same time, so cost-effective and processing quality analysis is necessary to determine which material is more economical.

PCBN tool material has better cutting performance than Al2O3.

When dry machining with Si3N4 hardened steel, the cost of Al2O3 ceramics is lower than that of PCBN materials.

Ceramic tools have good thermochemical stability, but not as good as the toughness and hardness of PCBN tools.

Ceramic tools are a good choice when cutting workpieces with hardness below 6 OHRC and small feed rates.

PCBN tools are suitable for workpiece hardnesses above 60HRC, especially for automated machining and high precision machining.

In addition, the residual stress on the surface of the workpiece after cutting the PCBN tool is relatively stable compared to the ceramic tool under the same flank wear.

The following principles should also be followed for dry-cut hardened steel using PCBN tools:

The maximum depth of cut is selected as much as possible under the rigidity of the machine tool, so that the heat generated in the cutting zone softens the metal in the front of the blade, which can effectively reduce the wear of the PCBN tool.

In addition, PCBN tools should be used whenever possible for small cuts.

Due to the poor thermal conductivity of the PcBN tool, the heat in the cutting zone is not too late, and the shear zone can also produce a significant metal softening effect, reducing the wear of the cutting edge.

Blade structure and geometrical determination

Proper determination of blade shape and geometry parameters is critical to maximizing tool cutting performance.

In terms of tool strength, the blade tip strengths of various blade shapes are from high to low: circular, 100° diamond, square, 80° diamond, triangle, 55° diamond, 35° diamond.

When the blade material is selected, the blade shape with high strength should be used as much as possible.

Hard turning inserts should also choose the largest possible tool nose arc radius, roughing with round and large radius inserts, and the tool nose radius during finishing is about 0.8 to 1.2 μm.

The hardened steel chips are red and soft forged strips, brittle, easy to break, and not bonded.

Generally, there is no built-up edge on the cutting surface, and the surface quality of the processing is high, but the cutting force of the hardened steel is relatively large, especially the radial cutting force is larger than the main cutting force.

Therefore, the tool should adopt a negative rake angle (go ≥ -5 °) and a large relief angle (ao = 10 ° ~ 15 °). The lead angle depends on the rigidity of the machine tool, generally taking 45 ° ~ 60 ° to reduce the workpiece and tool flutter.

Selection of cutting parameters

The higher the hardness of the workpiece material, the smaller the cutting speed should be.

The suitable cutting speed for hard turning finishing is 80-200 m/min, and the usual range is 10-150 m/min.

The cutting speed should be kept at 80-100m/min with large depth or strong intermittent work.

In general, the depth of cut is 0.1 to 0.3 mm.

When the surface roughness of the machined surface is high, a small cutting force can be selected, but it should not be too small and should be suitable.

The feed rate can usually be selected from 0.05 to 0.25 mm/r, depending on the surface roughness value and productivity requirements.

When the surface roughness is Ra 0.3 to 0.6 μm, hard turning is more economical than grinding.

Process system requirements

In addition to choosing a reasonable tool, hard turning has no special requirements for lathes or turning centers.

If the lathe or turning center is rigid enough and the required precision and surface roughness can be obtained when machining a soft workpiece, it can be used for the processing of hardened steel.

In order to ensure smooth and continuous turning operations, the usual method is to use rigid clamping devices and medium rake angle cutters.

However, it is generally believed that hard turning requires a highly rigid lathe, that is, the key to hard turning is that the machine has sufficient rigidity.

At the same time, the tool, the workpiece and the clamping device are compact in structure and have the same rigidity.

If the workpiece is positioned, supported and rotated under the cutting force, it can be kept fairly stable, and the existing equipment can be used for hard turning.

Hard turning application

After ten years of development and promotion, hard turning technology has achieved great economic and social benefits.

The following examples illustrate the promotion and application of hard turning technology in the production of roll processing industries.

Rolling bar processing industry

Large-scale roll enterprises have used hard turning technology to carry out cutting, roughing and finishing of various types of rolls such as chilled cast iron and hardened steel, and have achieved good results.

The average processing efficiency is increased by 2 to 6 times, and the processing time and power are saved by 50% to 80%.

For example, in the Wuhan Iron and Steel Company Rolling Mill, the cutting speed is increased by 3 times for turning and half finishing turning cast iron roll with a hardness of 60-80HS.

For turning every rollers, it saves electricity and labor costs by more than 400 yuan, saves nearly 100 yuan in tool costs, and has achieved huge economic benefits.

When FR22 cermet cutter was used to turn the 86CrMoV7 hardened steel roll of HRC58~63 (v=60m/min, f=0.2mm/r, ap=0.8mm), the single-blade continuous cutting roll path reached 15000m (VCmax=0.2mm) which meet the requirements of turning to replace grinding.

Industrial pump processing industry

At present, 70% to 80% of domestic pulp pump production plants have adopted hard turning technology.

The slurry pump is widely used in mining, electric power and other industries. It is an urgently needed product at home and abroad. Its sheath and shield are Cr15Mo3 high-hard iron castings of 63-67HRC.

In the past, it was difficult to turn it by various tools, so it was only necessary to use roughing after softening and then tempering.

After the hard turning technology, the problem of hardening processing was solved smoothly, and the two processes of annealing and quenching were eliminated, which saving a lot of man-hours and electricity.

Automotive processing industry

In the high-volume production industries such as automobiles and tractors, the machining problems of quenching hardware are often encountered in the crankshaft, camshaft and transmission shaft, knife measuring industry and equipment maintenance.

For example, in a locomotive and vehicle factory in China, the inner ring of the bearing needs to be processed in the maintenance of the equipment. The hardness of the inner ring of the bearing (material Gcr15) is 60HRC, and the diameter of the inner ring is 285mm. The grinding process is used, and the grinding allowance is uneven. 2h can be ground well; and with hard turning, one inner ring is processed in only 45 minutes.

Conclusion

After years of research and exploration, China’s hard turning technology has made great progress, but the application of hard turning technology in production is still not extensive.

The reasons are mainly as follows:

(1) The production enterprises and operators do not know enough about the effect of hard turning. It is generally believed that hard materials can only be ground;

(2) The tool cost is considered too high.

The initial tool cost of hard turning is more expensive than ordinary cemented carbide (such as CBN is more than 10 times more expensive than ordinary hard alloy), but the cost of each part is lower than that of grinding, which is more effective than ordinary hard alloy;

(3) Insufficient research on the mechanism of hard turning;

(4) Specifications for hard turning are not sufficient to guide production practices.

Therefore, in addition to in-depth study of the hard turning mechanism, it is necessary to strengthen the training of hard turning processing knowledge, successful experience demonstration and strict operating specifications, so that this efficient and clean processing method is more applied to production practice.

At present, if hard turning and fine grinding are combined, the cost of processing a general part will be 40% to 60% lower than the cost of roughing and finishing on the grinding machine.