AISI 304 austenitic stainless steel (i.e. 0Cr18Ni9 stainless steel) has good corrosion resistance, heat resistance, low temperature strength and comprehensive mechanical properties. It is widely used in food equipment, chemical equipment and atomic energy industrial equipment. This kind of austenitic stainless steel has good intergranular corrosion resistance, excellent corrosion resistance in many oxidizing acids (such as HNO3), strong corrosion resistance in alkali solution, most organic and inorganic acids, as well as in atmosphere, water and steam. AISI 304 austenitic stainless steel has a relative machinability Kr of about 0.4, which is a typical difficult cutting material.
Characteristics Of AISI 304 Stainless Steel Turning
AISI304 austenitic stainless steel has poor machinability, which is mainly manifested in large cutting force, serious work hardening, high local temperature in the cutting area and easy adhesion and wear of tools.
(1) Large Cutting Force
AISI 304 austenitic stainless steel has low hardness (hardness ≤ 187hbs) and good plasticity (elongation after fracture) due to its large amount of Cr, Ni, Mn and other elements δ 5 ≥ 40%, reduction of area ψ ≥60％)。 The plastic deformation during cutting is large, and the strength can still be maintained at high temperature (the strength of ordinary steel decreases significantly when the cutting temperature increases), resulting in a large cutting force of AISI304 austenitic stainless steel. Under conventional cutting conditions, the unit cutting force of AISI 304 stainless steel is 2450mpa, which is more than 25% higher than 45 steel.
(2) Severe Work Hardening
AISI 304 stainless steel is accompanied by obvious plastic deformation during machining, and the material lattice will produce serious distortion; At the same time, due to the defects of austenite structure in stability, a small part of austenite becomes martensite in this process; In addition, the impurity compounds in austenite will decompose due to heating as the cutting process proceeds, and the dispersed impurities will produce a hardened layer on the surface, which makes the work hardening phenomenon very obvious, and the strength after hardening σ B up to 1500MPa and the depth of hardened layer is 0.1-0.3mm.
(3) Local Temperature In Cutting Area Is High
Because AISI304 stainless steel requires large cutting force and the chips are not easy to be cut away, the work consumed by separating chips is also large. Cutting AISI 304 stainless steel under conventional conditions is about 50% higher than that of low carbon steel, resulting in more cutting heat. Austenitic stainless steel has poor thermal conductivity. The thermal conductivity of AISI304 stainless steel is 16.3-21.5w/m · K, which is only one third of the thermal conductivity of 45 steel. Therefore, the temperature of the cutting area is high (generally, the heat taken away by chips during cutting should account for more than 70% of the cutting heat). A large amount of cutting heat is concentrated on the cutting area and the “tool chip” contact surface, and the heat transferred into the tool is up to 20% (the value is only 9% when cutting general carbon steel), Under the same cutting conditions, the cutting temperature of AISI304 stainless steel is about 200-300 ℃ higher than that of 45 steel.
(4) The Tool Is Prone To Adhesion And Wear
Due to the high temperature strength and high work hardening tendency of austenitic stainless steel, the cutting load is heavy, and the affinity between austenitic stainless steel and tools and chips will be significantly enhanced due to the affinity between austenitic stainless steel and tools during the cutting process, which will inevitably produce bonding, diffusion and other phenomena, resulting in tool adhesion and wear. In particular, the hard inclusions formed by a small part of carbides accelerate the wear of tools, and even cause edge collapse, which greatly reduces the service life of tools and affects the surface quality of machined parts.
Choose A Reasonable Turning Process
Due to the poor machinability of AISI304 austenitic stainless steel, it is necessary to choose a reasonable turning process, including the reasonable selection of turning tool materials, tool geometric parameters, cutting parameters and coolant, in order to obtain higher production efficiency and processing quality.
(1) Tool Material
Correct selection of tool materials is of great significance to ensure efficient machining of austenitic stainless steel. According to the difficult turning characteristics of AISI 304 stainless steel, the analysis shows that the selected cutting tools should have the characteristics of high strength and toughness, at the same time, they should also have good wear and heat resistance, and ensure that they have little affinity with stainless steel. At present, cemented carbide and high speed steel are still the most commonly used cutting tool materials.
① Cemented carbide
Because the cutting force of difficult to machine materials is large and the contact between chips and the rake face is short, the cutting force is mainly concentrated near the edge, which is prone to edge collapse. Therefore, YG cemented carbide tools can be selected for machining. YG cemented carbide has good toughness, high wear resistance and red hardness, and good thermal conductivity. It is suitable for machining austenitic stainless steel. YG8N tool can also be selected. Due to the addition of Nb, the cutting performance is 1-2 times higher than YG8, and the effect is good when it is used in rough machining and semi precision machining.
② High speed steel
High speed steel tools can effectively avoid the phenomenon that hard tools are easy to be damaged due to the size, shape and structure of turning stainless steel workpiece. Traditional high-speed steel tools (such as W18Cr4V) can no longer meet the current processing requirements in terms of durability, but new high-speed steel tools with superior cutting performance such as aluminum containing high-speed steel (such as W6Mo5Cr4V2Al) and nitrogen containing high-speed steel (such as w12mo3cr4v3n) can be used.
(2) Tool Geometric Parameters
Reasonably determining the geometric parameters of the selected tool is an important factor to effectively improve the tool durability and the machining effect of AISI 304 stainless steel materials. Generally, the tool is required to have large front and back corners and sharp cutting edges.
(3) Cutting Parameters
AISI 304 stainless steel is a typical difficult to machine material, so the cutting parameters should be selected reasonably. Cutting parameters have great influence on work hardening, cutting force, heat and machining efficiency. Cutting speed has the greatest impact on cutting temperature and tool durability ν c. The second is the feed rate F, and the back feed rate AP has the lowest influence.
(4) Cutting Fluid
Due to the poor cutting performance of AISI304 stainless steel, the selected cutting fluid must have better cooling, lubricity and permeability (i.e. anti bonding performance). Emulsions and vulcanized oils containing extreme pressure additives such as s and Cl should be selected as far as possible.
Emulsion has good cooling performance and is mainly used for rough turning of stainless steel. Vulcanized oil has certain cooling and lubricating properties and low cost. It can be used for semi finishing or finishing of stainless steel. If extreme pressure or oily additives are added to the cutting fluid, its lubricating performance can be significantly enhanced. It is generally used for finishing machining of stainless steel. The cutting fluid made of the mixture of carbon tetrachloride, kerosene and oleic acid greatly improves the permeability of the cooling and lubricating fluid, and is especially suitable for the finishing machining of AISI 304 austenitic stainless steel materials. Due to the large cutting heat of austenitic stainless steel, spray cooling, high-pressure cooling and other methods should be used as far as possible to improve the cooling effect.