Hard turning is a machining process that involves turning hardened steel as the final or finishing step, as opposed to the more commonly used grinding technology.
Turning is a fundamental and crucial process in the manufacturing industry that has a significant impact on production efficiency, cost, energy consumption, and environmental sustainability.
The advancement of science and technology has led to the increased usage of various high-strength and high-hardness engineering materials. However, traditional turning techniques find it challenging or impossible to process these materials. Hard turning technology addresses this issue and provides substantial benefits in production.
Hardened steel is a material that has a martensite structure after undergoing a quenching process, and has high hardness, strength, and low plasticity. Its hardness is usually greater than 55 HRC, and its strength is around 2100 to 2600 N/mm2.
Before heat treatment, the workpiece undergoes roughing, and only the finishing is performed in the hardened state.
Fine grinding is the commonly used finishing process, but it has limitations such as a narrow processing range, high investment cost, low production efficiency, and a tendency to cause environmental pollution.
With the progression of processing technology, hard turning has emerged as a viable alternative to grinding and has achieved considerable benefits in production. Currently, hardened steel with a hardness ranging from 55 to 65 HRC can be machined using polycrystalline cubic boron nitride (PCBN) tools, ceramic tools, or coated carbide tools on a lathe or turning center.
Hard Turning Features
High Processing Efficiency
Hard turning boasts higher processing efficiency compared to grinding, using only 1/5 of the energy required for conventional grinding. This is due to the use of larger cutting depths and higher workpiece rotation speeds, resulting in a metal removal rate that is typically 3-4 times higher than grinding.
In the turning process, multiple surface processing tasks, such as outer circle, inner hole, slot, etc., can be completed in one clamping, while grinding requires multiple setups. This results in reduced auxiliary time and high positional accuracy between machined surfaces.
Hard Turning is a Clean Processing Process
In most instances, hard turning does not require the use of coolant. In fact, the use of coolant can have a negative impact on tool life and surface quality. This is because hard turning involves making the material of the sheared portion soft and annealed.
If the cooling rate is too high, the effect of the cutting force is diminished, leading to increased mechanical wear and shortened tool life.
At the same time, hard turning eliminates the need for coolant-related equipment, reducing production costs, simplifying the production system, and producing chips that are clean and easy to recycle.
Low Equipment Investment, Suitable for Flexible Production Requirements
Compared to grinding machines, lathes have lower investment costs, with a 1/3 to 1/2 investment ratio for the same productivity. Additionally, the cost of the auxiliary system is also lower.
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 repeatability.
The lathe itself is a flexible processing method with a wide range of capabilities. Clamping and turning is fast, and modern CNC lathes equipped with multiple tool turntables or magazines make it easy to switch between two different workpieces, making hard turning ideal for this type of machining.
Therefore, hard turning is more suitable for flexible production requirements than grinding.
Hard Turning Enhances Overall Machining Accuracy
In hard turning, most of the heat generated is carried away by the chips, which prevents surface burns and cracks that are commonly seen in grinding. This results in excellent surface quality and precise roundness, ensuring high positional accuracy between machined surfaces.
Hard turning tool materials and the selection
Coated cemented carbide
Coated cemented carbide tools are equipped with one or multiple layers of wear-resistant coatings, such as TiN, TiCN, TiAlN, and Al2O3, on a carbide tool that has undergone toughness hardening. The coating thickness ranges from 2 to 18μm.
The coatings serve two main purposes:
Firstly, it has a much lower heat transfer coefficient compared to the tool base and workpiece material, reducing the thermal impact on the tool base. Secondly, it can effectively enhance the friction and adhesion during the cutting process, and minimize the generation of cutting heat.
Compared to uncoated cemented carbide tools, coated carbide tools offer significant enhancements in terms of strength, hardness, and wear resistance.
Low-cost coated carbide can achieve high-speed turning for workpieces with a dry hardness between 45 and 55 HRC.
In recent years, some manufacturers have improved the performance of coated tools by refining the coating materials and proportions. For instance, some US and Japanese manufacturers use Swiss AlTiN coating materials and patented coating technology to produce blades with a hardness of 4500 to 4900 HV. The hardness remains unchanged and does not oxidize even when the turning temperature is as high as 1500°C to 1600°C.
These blades have a life span that is four times longer than that of general coated blades. They are also cost-effective, with a cost that is only 30% of the general blades, and have good adhesion. These blades can process die steel with a hardness of 47-52HRC at a speed of 498.56m/min.
Ceramic tools are renowned for their high hardness (91 to 95HRA), strength (bending strength of 750 to 1000 MPa), wear resistance, chemical stability, anti-blocking performance, low friction coefficient, and affordable price.
When used under normal conditions, ceramic tools have exceptional durability and can be 2 to 5 times faster than cemented carbide tools. They are particularly suitable for processing high hardness materials, finishing, and high-speed machining. They can handle all types of hardened steel and cast iron with a hardness of 62HRC.
Common ceramic materials used are alumina-based ceramics, silicon nitride-based ceramics, cermets, and whisker-toughened ceramics.
In recent years, through extensive research and the implementation of new manufacturing processes, the flexural strength and toughness of ceramic materials have significantly improved. For example, the new cermet NX2525 developed by Mitsubishi Metal Corporation in Japan, and the new CT series and coated cermet blade series of cermet inserts developed by Sandvik Coromant in Sweden.
The grain structure of these materials has a diameter as small as 1 μm or less, resulting in higher flexural strength and wear resistance compared to ordinary cermets, thereby expanding the application range of ceramic materials.
The silicon nitride ceramic material tool developed by Tsinghua University has also reached an advanced level globally.
Cubic Boron Nitride (CBN) is second only to diamonds in terms of hardness and wear resistance, and it also has exceptional high temperature hardness.
Compared to ceramic tools, CBN has slightly lower heat resistance and chemical stability, but it has better impact strength and crush resistance.
It is widely used for cutting hardened steel (above 50HRC), pearlitic gray cast iron, chilled cast iron, and high temperature alloys.
CBN tools can increase the cutting speed to a higher level compared to carbide tools.
PCBN tools with high CBN content have high hardness, good wear resistance, high compressive strength, and good impact toughness. However, they have poor thermal stability and low chemical inertness, making them suitable for cutting heat-resistant alloys, cast iron, and iron-based sintered metals.
The composite PCBN tool has a low CBN particle content and uses ceramics as a binder. Its hardness is lower, but it compensates for the poor thermal stability and low chemical inertness of pure PCBN, making it suitable for cutting hardened steel.
In the field of cutting gray cast iron and hardened steel, both ceramic and CBN tools can be considered, and a cost-effective and processing quality analysis is necessary to determine which is more economical.
PCBN tools have better cutting performance than alumina ceramics (Al2O3). However, for dry machining with silicon nitride hardened steel, the cost of Al2O3 ceramics is lower than that of PCBN materials.
Ceramic tools have good thermochemical stability, but they are not as tough and hard as PCBN tools. Ceramic tools are a good choice when cutting workpieces with hardness below 60HRC and small feed rates.
PCBN tools are suitable for workpiece hardnesses above 60HRC, especially for automated and high precision machining. Furthermore, the residual stress on the workpiece surface after cutting with a PCBN tool is relatively stable compared to that of a ceramic tool under the same flank wear.
The following principles should be followed for dry cutting hardened steel using PCBN tools:
The maximum depth of cut should be 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 front of the blade, reducing the wear of the PCBN tool.
In addition, small cuts should be made with PCBN tools whenever possible.
Due to the poor thermal conductivity of the PCBN tool, the heat in the cutting zone is not easily dissipated, and the shear zone can also produce significant metal softening, reducing the wear of the cutting edge.
Blade structure and geometrical determination
Determining the proper blade shape and geometry parameters is essential for maximizing cutting performance of the tool.
In terms of tool strength, the blade tip strengths of various blade shapes range from highest to lowest: circular, 100° diamond, square, 80° diamond, triangle, 55° diamond, and 35° diamond. When selecting the blade material, it is advisable to choose the blade shape with high strength as much as possible.
For hard turning inserts, it is best to choose the largest possible tool nose arc radius. For roughing, round and large radius inserts should be used, and the tool nose radius during finishing should be about 0.8 to 1.2 μm.
Hardened steel chips are brittle and tend to break easily, and are not bonded. They are typically red and soft, like forged strips. The cutting surface does not generally have a built-up edge, and the surface quality of the processed workpiece is high, but the cutting force of the hardened steel is relatively large, with the radial cutting force being larger than the main cutting force.
Therefore, the tool should have a negative rake angle (go ≥ -5°) and a large relief angle (ao = 10° to 15°). The lead angle depends on the rigidity of the machine tool, generally taking 45° to 60° to reduce workpiece and tool flutter.
Selection of cutting parameters
The hardness of the workpiece material should dictate the cutting speed. The more significant the hardness of the material, the lower the cutting speed should be.
The suitable cutting speed for hard turning finishing is between 80 and 200 m/min, with the typical range being 10 to 150 m/min.
When working with large depths or strong intermittently, it is best to keep the cutting speed at 80 to 100 m/min.
In general, the depth of cut ranges from 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 appropriate.
The feed rate can typically be chosen from 0.05 to 0.25 mm/r, depending on the surface roughness value and productivity requirements.
If the surface roughness is between Ra 0.3 and 0.6 μm, hard turning is more cost-effective than grinding.
Process system requirements
Hard turning does not have specific requirements for lathes or turning centers, beyond selecting a suitable tool.
If a lathe or turning center is rigid enough and can achieve the desired precision and surface roughness when machining a soft workpiece, it can be used for processing hardened steel.
To ensure smooth and continuous turning operations, rigid clamping devices and medium rake angle cutters are typically used.
However, it is widely believed that hard turning requires a highly rigid lathe, meaning that the key to hard turning is machine rigidity.
At the same time, the tool, workpiece, and clamping device should be compact in structure and have comparable rigidity.
If the workpiece is positioned, supported, and rotated under the cutting force, it can be kept stable, and existing equipment can be used for hard turning.
Hard turning application
After ten years of development and promotion, hard turning technology has produced significant economic and social benefits. The following examples demonstrate the application of this technology in roll processing industries.
Roll Processing Industry:
Large roll companies have utilized hard turning technology for cutting, roughing, and finishing various types of rolls, including chilled cast iron and hardened steel, with excellent results. The average processing efficiency has increased by 2 to 6 times, resulting in a 50% to 80% reduction in processing time and power.
For example, at the Wuhan Iron and Steel Company Rolling Mill, the cutting speed for turning and half finishing turning cast iron rolls with a hardness of 60-80HS has increased by 3 times. This has resulted in savings of over 400 yuan in electricity and labor costs and nearly 100 yuan in tool costs, delivering significant economic benefits.
When using an FR22 cermet cutter to turn a 86CrMoV7 hardened steel roll with a hardness of HRC58~63 (v=60m/min, f=0.2mm/r, ap=0.8mm), a single-blade continuous cutting roll path of 15000m (VCmax=0.2mm) was achieved, which meets the requirements for turning instead of grinding.
Pump Processing Industry:
Currently, 70% to 80% of domestic pulp pump production plants have adopted hard turning technology. The slurry pump, which is widely used in industries such as mining and electric power, is a highly sought-after product both domestically and abroad. Its sheath and shield are made of Cr15Mo3 high-hard iron castings with a hardness of 63-67HRC.
In the past, it was challenging to turn using various tools, so it was necessary to soften and temper before roughing. However, with hard turning technology, the problem of hardening processing has been resolved, and the processes of annealing and quenching have been eliminated, saving significant man-hours and electricity.
Automotive Processing Industry:
In high-volume production industries such as automobiles and tractors, machining problems often arise with quenched hardware in crankshafts, camshafts, transmission shafts, knife measuring equipment, and maintenance. For instance, in a locomotive and vehicle factory in China, the inner ring of a bearing needs to be processed during equipment maintenance.
The hardness of the inner ring (made of material Gcr15) is 60HRC and its diameter is 285mm. In the past, the grinding process was used, but the grinding allowance was uneven, taking 2 hours to grind well. With hard turning, one inner ring can be processed in just 45 minutes.
After years of research and exploration, China’s hard turning technology has seen significant advancements. However, its application in production remains limited.
The main reasons for this are:
(1) Lack of knowledge among production enterprises and operators regarding the benefits of hard turning. There is a widespread belief that hard materials can only be ground;
(2) High tool cost. The initial cost of hard turning tools is more expensive than ordinary cemented carbide, with CBN being over 10 times more expensive. However, the cost per part is lower than grinding and more effective than ordinary hard alloys;
(3) Insufficient research on the mechanics of hard turning;
(4) Inadequate specifications for hard turning to guide production processes.
Therefore, to increase the application of this efficient and clean processing method, it is necessary to deepen the study of the hard turning mechanism, enhance training on hard turning processing, showcase successful experiences, and establish strict operating specifications.
Currently, combining hard turning and fine grinding can result in processing costs that are 40% to 60% lower than roughing and finishing on a grinding machine for a typical part.