Wet and Dry Cutting Battle: The Controversial White Layer

Fill in the pits and fill the pits, "Sweep the whole article, take you to see through and understand PCBN" mentioned at the end of the article to bring you an article introducing the characteristics of PCBN tools and dry and wet cutting applications. To this end, the author read some literature and finally selected "Surface Integrity of PCBN Tool Dry and Wet Cutting Hardened Steel": This paper analyzes the dry and wet cutting by comparing the "white layer" of the workpiece section and the change of surface roughness. difference.

1. The battle of pros and cons

common view

It is generally believed that hard dry cutting of PCBN tools, on the one hand, can give full play to the excellent cutting performance and metal softening effect of the tool, and on the other hand, saves the use of cutting fluid during processing (usually cutting fluid accounts for 16% of the total processing cost. -20%), which brings huge economic benefits to enterprises, and is beneficial to environmental protection and workers' health. It is an ideal processing method.

However, while dry cutting brings many advantages, we should also see that in the harsh environment of dry cutting, both the workpiece and the tool are subjected to high mechanical and thermal loads, and the surface quality of the workpiece will inevitably be affected by the cutting temperature. Therefore, in order for dry cutting technology to be fully accepted by people, the surface quality it produces must exceed or be equivalent to that of wet cutting.

2. Application background

Whether it is dry cutting or wet cutting, the only indicators for judging the performance are tool life and machining quality. That is to say, the length of tool life and the quality of the surface of the workpiece are two important factors that determine whether hard cutting technology can be widely used.

Among these two factors, the importance of surface quality is more prominent.

Studies have found that even an acceptable rate of tool wear can result in an unacceptable surface finish.

Especially in the field of hardened steel processing, the residual stress state of the workpiece surface and the thickness of the "white layer" are the main evaluation criteria for tool change. It is more important than surface roughness or machining accuracy.

However, these parameters are difficult to measure in manufacturing enterprises, so it is very important to better understand the manifestation of white layer and its influencing factors in actual production.

In order to better understand the difference of surface quality between dry and wet cutting, this paper takes PCBN tool dry and wet cutting hardened steel GCr15 as an experiment to compare the surface quality of the machined.

Before explaining, it is necessary to introduce a concept that often occurs when PCBN tools cut hardened steel and other materials: white layer.

3. White layer

What is the white layer?

The white layer is usually harder and more brittle than the matrix structure, and appears white under an optical microscope, so it is called "white layer".

At present, there is no consensus on the organizational composition of the white layer. Some foreign studies have pointed out that the white layer is a combination of untempered martensite (α) and over-tempered austenite (γ). Some scholars also believe that retained austenite is the main component of the white layer structure, and it has been proved by experiments that the volume fraction of retained austenite has increased by about three times compared with the original structure.

In addition, some people think that the microstructure of the white layer is an irregular martensite, which is composed of tiny grains, the workpiece material has a high density, and the distribution of carbides is relatively uniform.

Why is the white layer generated?

The author of the original text believes that the formation of the white layer is the result of the combined action of three reasons:

1. Rapid heating and quenching lead to material transformation (dominant role).

During dry cutting, the cutting temperature is extremely high, especially after the tool is worn, the cutting edge becomes blunt, the extrusion effect in front of the edge area increases, and the clearance angle of the tool becomes zero, the friction between the workpiece and the tool increases, making the cutting The temperature is significantly increased, and the local temperature of the workpiece surface has reached or exceeded the equilibrium transformation temperature, and the surface of the workpiece to be processed is transformed into austenite state.

When the tool leaves, because the heat is dissipated into the air and transferred into the workpiece material, the surface of the workpiece is rapidly cooled, so that the temperature at which martensite is formed is reached at this time, but due to the short time, some austenite has not had time to transform, This process has ended.

2. Severe plastic deformation leads to grain refinement.

The extrusion action of the chamfered tool refines the grains on the surface of the workpiece, which may also be one of the reasons for the formation of white layers.

3. Surface nitriding.

During dry cutting, the cutting temperature is very high, and the workpiece is exposed to the air where the tool passes, so the high temperature workpiece

The surface is very easy to react chemically with air, while wet cutting is protected by cutting fluid and the temperature is relatively low, which inhibits the occurrence of chemical reactions.

White layer effect?

The influence of the white layer on the service life of the workpiece has always been controversial.

On the one hand: the high hardness surface and thermal stability of the white layer are considered to be tribological advantages, and the gradual transformation of the microstructure can increase the wear resistance. Therefore, some scholars believe that the white layer increases the surface hardness and has high thermal stability. The degree of abrasive wear is increased, and the wear resistance is increased.

On the other hand, microscopic cracks at different angles from the surface are commonly found in the white layer. Because the white layer is hard and brittle, the cracks are easy to concentrate and expand inside, resulting in large pieces of workpiece material falling off in the form of delamination.

In addition, the white layer is usually related to tensile stress, which is not conducive to the fatigue resistance of the surface. Some scholars have pointed out that the white layer reduces the bending fatigue strength, which may be related to the residual tensile stress.

Before discussing dry and wet cutting, the author first introduces the concept of white layer, because white layer is a common feature of PCBN hard cutting, which is affected by cutting temperature and is the best reference for discussing the effect of dry and wet cutting on workpiece.

In the next article, the author will introduce the experimental procedure and its conclusions.

References: Cao Yongquan, "Research on Surface Integrity of Hardened Steel in Dry and Wet Cutting of PCBN Tools"