Factors affecting the machinability of gray cast iron

Iron carbon alloys with carbon content greater than 2.14% or eutectic structure in the structure are called cast iron. It is not a simple iron carbon binary alloy, but a multicomponent alloy dominated by iron carbon silicon. Gray cast iron is cast iron in which most or all of the carbon is precipitated as flake graphite and the section is dark gray after solidification. The structure of gray cast iron is that graphite is distributed on the metal matrix. The shape, size, quantity and distribution of graphite have an important impact on the properties of gray cast iron.

The degree of graphitization determines the structure of gray cast iron. The so-called graphitization is the process of carbon atom precipitation and Graphite Formation in cast iron. Graphite can be precipitated from liquid or decomposed from cementite. In order to obtain the desired microstructure, the key is to control the degree of graphitization. The unique constituent phase of gray cast iron is graphite. Gb7216-1987 divides graphite into 6 types according to different shapes, including flake A-type graphite, chrysanthemum B-type graphite, flake C-type graphite, dendritic point D-type graphite, dendritic flake E-type graphite and star F-type graphite.

In the actual production, type a graphite, type B graphite and type C graphite appear more. In the production of gray cast iron cylinder block, due to the uniform distribution of flake graphite, the mechanical properties and processability are very good, so the A-type graphite form of gray cast iron is the pursuit in production. Gb7216-1987 divides gray cast iron matrix into 7 types according to its as cast or after heat treatment: ferrite, flake pearlite, granular pearlite, troostite, granular bainite, acicular bainite and martensite. Generally, when the cooling rate is low, there are four kinds of as cast microstructure: ferrite, flake pearlite, granular pearlite and troostite, especially ferrite and flake pearlite. Therefore, controlling the amount, distribution and shape of ferrite and flake pearlite is very important for the properties of gray cast iron.

The cutting performance of gray cast iron workpiece refers to the difficulty of cutting the workpiece material under certain conditions. There are many factors affecting the cutting performance, such as uneven machining allowance, large hardness difference, quench structure on the surface, etc. However, it is found that the main problem is the casting material. There is no clear cause analysis and effective measures to improve the machinability of domestic gray cast iron castings and foreign high-quality gray cast iron castings.

There are many indicators to measure the machinability of gray cast iron, which can be studied by cutting force, cutting heat and tool wear. The measuring device and process of cutting force are relatively simple and direct, which can dynamically study the changes of cutting performance in the cutting process of gray cast iron; The measuring device of cutting heat of gray cast iron is complex, and is greatly affected by the test environmental factors, which has large errors. It is generally used to supplement the other two evaluation indexes; Tool wear is the most direct way to evaluate the cutting performance of materials, especially when various factors are stable. However, tool wear evaluation belongs to static evaluation, which can not evaluate and analyze the changes of cutting performance in the machining process in real time. Moreover, the material consumption of the test is large and the test cycle is long.

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