Features of stainless steel processing

In modern installations and aggregates, working mechanisms are subjected to significant loads on all structural elements. The operation of parts at high speeds, pressure and temperatures leads to the fact that elements made of ordinary steels quickly fail. To work in difficult conditions, special alloys are needed, which include stainless steel. Stainless steel refers to alloy steels that are resistant to corrosion, both in the atmosphere and in aggressive environments. "Stainless steel" is called steel with a chromium content of more than 12%. With an increase in the amount of chromium, the corrosion resistance of steels increases. To obtain the necessary structure and mechanical properties, stainless steels are additionally alloyed with nickel and molybdenum.

Stainless steels can be divided into the following main groups:

• Ferritic stainless steels often have high strength and are well processed by cutting. • Martensitic stainless steels are processed normally.
  
• Austenitic stainless steels are characterized by a high elongation coefficient. They are difficult to process.
  
• Austenitic ferritic stainless steels are often also called duplex stainless steels. They also belong to the hard-to-work.
  

The machinability of stainless steels in most cases is low and very much depends on the composition of alloying elements, as well as heat treatment, and even the method of obtaining the workpiece (casting, forging, etc.). These factors give stainless steels features that make their machining difficult.

What are these features:

• Most stainless steels are self-hardening during deformation, including when removing chips. The degree of riveting decreases as you move away from the place of deformation. In the cutting area, the hardness can increase by 100%, especially if the tool is selected incorrectly. Grooves appear on the tool
  
• The high strength of these steels leads to heavy loads on the cutting edge tool. Together with poor thermal conductivity and riveting, this causes low machinability of these steels by cutting.
  
• Stainless steels are prone to sticking to the surface of the cutting tool.
  
• Difficulties with chip breaking and burr formation due to high strength are also one of the features of these steels
  
• Low thermal conductivity. Poor ability to remove heat leads to an increase in temperature in the cutting area, which affects tool wear. This is one of the main factors complicating the processing of stainless steels.

These features do not allow to accelerate when processing stainless steel, it is necessary to reduce the speed and feed to small values to ensure more or less normal tool durability and achieve the desired surface quality. Nevertheless, ”stainless steel” is an excellent material that is widely used in mechanical engineering.

To summarize, the main recommendations that need to be taken into account when processing stainless steel are:

• When turning, choose the largest possible radius at the top of the plate
• Be sure to use СОЖ with precise direction to the cutting area and preferably under pressure, and the higher the pressure, the better. Since the processing of stainless steel and especially austenitic steels leads to the release of a large amount of heat. This is a key factor that negatively affects the processing, heat is not removed and transferred to the tool.
• For drilling, use drills with internal coolant supply. This will keep the minimum degree of self-hardening (within 10%) of stainless steel during processing.
  
• At higher feed rates, heat removal from the cutting area is greater. It is necessary to remember this when eliminating the difficulties that arise when processing stainless steel.
  
• When setting the cutting speed, always start with the lower recommended values, because different batches of workpieces can be processed in different modes. Also keep in mind that for deeper holes it is necessary to reduce the speed by 10-20% of the recommended values.
  
• When threading in duplex and high-alloy stainless steels, use the lower values of the recommended ranges.
  
• Use mineral oil if possible; if you have to use an emulsion, its concentration should be at least 8%.
  
• The first choice when processing stainless steel is a coated tool, because it better prevents the formation of a build-up.
  
• Do not use a worn-out tool, because this increases the slope of the material during processing and leads to tool breakage.

Information used: recommendations of tool manufacturers Dormer Pramet, Sandvik, Kennametal.