【Technology】Hard Turning

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.

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.

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.

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.