Variations in material properties of grey cast iron and its impact on tool wear

Abstract

Among cast materials, grey cast iron and ductile iron makes up about three quarters of all cast parts. This makes the manufacturing and machining of grey cast iron an important economical factor for the automotive industry.

In a world with finite resources and growing concerns for the environment the importance and interest in recycling grows larger. This has led the Volvo Trucks Corporation to use a higher quantity of recycled metal in their casted parts. The increase in recycled metal causes an increase in the quantity of impurities and trace elements, which could potentially lower the machinability of the grey cast iron used in the engine blocks of Volvo trucks.

Currently, the Volvo Trucks Corporation experience a high unpredictability of the tool wear when machining engine blocks, this study investigates how the properties of the workpiece material differs between manufactured series of engine blocks and how this impacts the degree of tool wear. This was done by collecting and examining of cutting tools from a rough milling operation in one of Volvos production lines as well as examining material samples from engine blocks machined by said cutting tools.

The evaluated workpiece material properties were the chemical content, the graphite structure, the hardness, and the concentration of hard inclusions. The material properties were put in relation to the degree of tool wear.

The results shows that there was considerable variations in the degree of tool wear between the studied series, ranging from virtually undamaged to severe fractures. Severe degree of tool wear had a strong correlation with a high concentration of inclusions. The concentration of inclusions were in turn higher in samples with a high content of trace elements. The result also shows that a large number of material samples had an undesired graphite distribution of type D and E instead of type A. The hardness of the workpiece material was fairly consistent in all samples but showed a weak correlation between high hardness and graphite distribution of type E or D. Harder materials were in general easier to machine, which probably can be explained by a lower concentration of inclusions.

Popular Abstract

The problems with recycled metals in the manufacturing industry

Recycling of metals has become increasingly important for the manufacturing industry. What new problems does this cause, and does the lowered cost of material in the start of the production chain compensate for the difficulties in machining recycled materials?

Casted materials are everywhere around us. Most of us uses casted products, or products that have casted parts, every day. Maybe you start your day by eating some eggs that you fried in a cast iron skillet and then take the bus, whose engine are made of a large number of casted parts, to your work or campus. Moreover, if you by chance do not use a cast product you will most certainly use a product that been manufactured by machines that have casted parts.

Nowadays, casting are used as a manufacturing method for both plastics and metals, but it is still metal that are most closely associated with casting. A wide range of metals and alloys can be casted, the most common is iron. Cast iron in turn can be anyone of several iron-carbon alloys, including grey cast iron, white cast iron, ductile cast iron, and etcetera. Grey cast iron is the most popular; it constitutes 43 % of the world’s total production of casted metals. It gains its popularity from a number of desirable qualities, including low cost, ease of machining, excellent wear resistance and good damping capacities.

In recent years, growing concern for the environment have caused the casting industry to use a higher proportion of recycled material in their casted goods. This in turn have led to an increase of undesired trace elements and impurities in the casted material. Lately, the Volvo Trucks Group in Skövde, Sweden has experienced a high variation of wear on their cutting tools when they machine engine blocks made of grey cast iron. They suspected that the increase in recycled material, and consequently in trace elements, has led to an increase in undesired titanium carbonitrides, Ti(C,N), inclusions that subjects the cutting tools to a high degree of wear.

In this study on the subject, the concentration of Ti(C,N) was compared to the material properties hardness and graphite structure. These three factors were then compared to the degree of tool wear on cutting tools used to machine gray cast iron engine blocks.

The findings indicates that a high level of titanium in the casted material gave rise to a high concentration of Ti(C,N)-inclusions. A high concentration of such inclusions were strongly correlated to a high degree of tool wear on the cutting tools used. The Ti(C,N)-inclusions are extremely hard, even harder than the thin coating of aluminum oxides that are applied to cutting tools. When the tool cuts through the grey cast iron and hits a Ti(C,N)-inclusions it will subject the cutting edge to an abrasive wear that gradually will deteriorate it and finally weaken it in such a manner that fracture will occur.

Interestingly, and somewhat contra-intuitive, analyzed materials that had a high hardness did not give rise to a high degree of cutting tool wear and vice versa. This is explained by the tendency of inclusions to acts as fracture initiator and stress concentrator on a micro-level in the casted materials. Thus, low concentration of inclusions causes a casted material to be harder, and a high concentration softens the material, causing harder materials to be easier to machine.

• author: Söderholm, Andreas
• supervisor: Christina Windmark; Rebecka Lindvall
• organization: Production and Materials Engineering
• course: MMTM01 20202
• year: 2021
• type: H3 - Professional qualifications (4 Years - )
• subject: Technology and Engineering
• keywords: Grey cast iron, hard inclusions, tool wear, machinability, graphite structure.
• report number: CODEN:LUTMDN/(TMMV-5309)1-102/2021
• language: English
• id: 9042297
• date added to LUP: 2021-03-23 15:05:48
• date last changed: 2021-03-23 15:05:48