GB2148939A

GB2148939A - A spheroidal graphite cast iron workpiece cast in sand

Info

Publication number: GB2148939A
Application number: GB08424688A
Authority: GB
Inventors: Dr Diethard Schock
Original assignee: Sulzer AG; Gebrueder Sulzer AG
Current assignee: Sulzer AG
Priority date: 1983-10-31
Filing date: 1984-10-01
Publication date: 1985-06-05
Also published as: FR2554022A1; CH655950A5; FR2554022B1; JPS6096747A; AT389712B; ATA211384A; GB8424688D0; DE3339885C1; GB2148939B

Abstract

To obtain improved plastic deformation properties in spheroidal graphite cast iron workpieces and to avoid precipitation of secondary graphite during martensite hardening and tempering, the workpiece which contains not more than 1% by mass of Si, is free from Al and its structure in the cast state is carbide-free. After heat treatment at least one of the following basic structures is present: ferrite, pearlite, bainite and martensite. These properties are obtained by performing, shortly before solidification - i.e. after the metal leaves the pouring ladle, e.g. in the mould, stream of metal or pouring basin - an inoculation treatment during which 0.1-0.2% by mass of silicon is added to the melt. Solidification is followed by heat treatment to product the aforementioned structure. The iron may contain, in wt.% vanadium 0-0.5 chromium 0-0.5 manganese 0-1 nickel 0-10 copper 0-3 molybdenum 0-1 %

Description

SPECIFICATION A spheroidal graphite cast iron workpiece cast in sand This invention relates to a workpiece cast in sand, having wall thicknesses greater than 10 mm, made of spheroidal graphite cast iron containing up to 1.0% by mass of silicon (Si), preferably 0.2-0.6% Si.

Cast iron containing up to 1 % by mass of silicon (all percentages are to be understood as by mass) is described in CH-A-508 729. In order to precipitate carbon as graphite during solidification, the cast iron contains 1-3% aluminium (Al) which replaces part of the silicon as a graphitizing element. As is known, silicon and aluminium increase the brittleness of the material structure. To obtain maximum toughness, therefore, it is necessary to keep the Si and Al content in the cast iron at a minimum.

According to one aspect of the present invention, a workpiece cast in sand, having wall thicknesses greater than 10 mm, made of spheroidal graphite cast iron containing not more than 1.0% by mass of silicon (Si), is characterised in that it is free from aluminium, its structure in the cast state is carbide-free, and after heat treatment it has at least one of the following basic structures: ferrite, pearlite, bainite and martensite. The term "free from aluminium" is intended to mean free from aluminium other than aluminium present as an impurity and thus refers to an aluminium content of less than 0.1%. The invention thus provides spheroidal graphite cast iron having an Si content of not more than 1 % without adding another graphite-forming element, more particularly no Al.

According to a second aspect of the present invention, a method of producing a workpiece according to the first aspect is characterised in that after Mg treatment required for forming spheroidal graphite, an inoculating alloy is added to the melt shortly before solidification so as to add 0.1-0.2% by mass of Si to the melt, and solidification is followed by heat treatment to obtain the aforementioned structure. By "shortly before solidification" is meant that inoculation is carried out after the metal leaves the pouring ladle, e.g. in the mould, in the stream of metal during pouring or in the pouring basin or dish runner. This relatively late inoculation ensures that graphite is precipitated, even without the conventional quantities of graphitizing elements.

Advantageously Si is added by adding 0.15 to 0.3% ferrosilicon containing 80% Si (FeSi-80) to the melt. The treatment of the melt with Mg to obtain spheroidal graphite can be done with pure Mg or Ni-Mg or Cu-Mg or a high-percentage FeSi-Mg.

In the cast state the workpiece usually has a pearlitic structure without primary carbides. The required structures are obtained by heat treatment, as is known for producing these structures.

To this end the workpiece is heated, e.g. from cast state, to the austenite range, i.e. from 700 to 950"C, is held in this range for 0.25 to 10 hours and is then cooled at the speed required for the desired structure. A ferrite structure can be obtained by slow cooling in a furnace at a maximum of 20'C/h, whereas pearlite is produced by cooling in air. Bainite is obtained by isothermic conversion in a salt, oil or lead bath between 250 and 450"C.

An additional requirement is to use the aforementioned low Si cast iron to produce a workpiece which can be given a martensite tempered and hardened structure by tempering at various temperatures without precipitating secondary graphite. To this end, in order to produce a martensite hardened and tempered structure having a given strength and ductility, the workpiece may be quenched from the austenite range in air, oil or water and then tempered at a temperature between 400 and 720"C depending on the desired strength and ductility.

A given combination of strength and ductility can be obtained by suitably choosing the tempering temperature.

The invention will now be explained in detail with reference to an Example, in which a workpiece is produced with a pure martensite hardened and tempered structure.

Equal quantities of low-silicon pig iron and unalloyed steel scrap, each having a maximum Si content of 0.2%, were melted in an induction furnace having an acid or basic lining, adding a recarburizer such as graphite to obtain the desired carbon content in the final composition of the melt given hereinafter, and heated to a temperature of 1550on.

At this temperature the melt was treated with Mg to obtain spheroidal graphite, by adding 0.5% of the additional alloy FeSi-Mg-40 (containing about 40% Mg) to the melt by immersion.

Next, the melt treated with Mg was poured into a sand mould to produce a cast article; inoculation was performed during the casting process, the inoculating agent being 0.2% FeSi80, containing 80% Si.

A sample of the cast melt was taken for analysis and used to determine the following final composition of the melt (in % by mass): C 3.4-3.7 Si 0.4-0.6 Mn 0.1-0.2 Mg 0.04-0.06 P max. 0.08 S max. 0.01 After cooling in the cast state, the workpiece had a pearlitic structure containing no primary carbides.

Heat treatment was first performed by heating to 900 C in the austenite range, i.e. between about 720 and 1 145"C. The workpiece was kept at the aforementioned temperature of 0.25 h, although in general holding times of 0.1 to 1 hour are permissible. From the austenite range, the workpiece was quenched in oil, producing a martensite structure.

This was followed by tempering at temperatures of 400 to 720'C-the temperature is chosen in known manner depending on the desired mechanical properties, more particularly strength and plasticity-followed by cooling in a furnace of air. After the described heat treatment, the structure of the workpiece was a pure martensite hardened and tempered structure free from secondary graphite precipitates. Its mechanical properties depending on the tempering temperature were: 0.2% proof stress Rpo2 N/mm2 300-900 Tensile strength R, N/mm2 500-1 200 Breaking elongation A5 % 1 2-2 Brinell hardness HB 200-350 Notch toughness Av J 1 2-2 While the invention can be applied to cast irons having various alloying ingredients the following ranges are preferred: Vanadium (V): 0-0.5% by mass Chromium (Cr): 0-0.5% by mass Manganese (Mn): 0-1.0% by mass Nickel (Ni) 0-10% by mass Copper (Cu): 0-3.0% by mass Molybdenum (Mo): 0-1.0% by mass.

Claims

1. A workpiece cast in sand, having wall thicknesses greater than 10 mm, made of spheroidal graphite cast iron containing not more than 1.0% by mass of silicon (Si), characterised in that it is free from aluminium, its structure in the cast state is carbide-free, and after heat treatment it has at least one of the following basic structures: ferrite, pearlite, bainite and martensite.

2. A workpiece as claimed in Claim 1 which has, apart from impurities, the following alloying elements: Vanadium (V): 0-0.5% by mass Chromium (Cr): 0-0.5% by mass Manganese (Mn): 0-1.0% by mass Nickel (Ni): 0-10% by mass Copper (Cu): 0-3.0% by mass Molybdenum (Mo): 0-1.0% by mass.

3. A workpiece as claimed in Claim 1 or Claim 2 which contains from 0.2 to 0.6% Si by mass.

4. A workpiece substantially as described herein with reference to the foregoing Example.

5. A method of producing a workpiece as claimed in any of Claims 1 to 3, characterised in that after Mg treatment required for forming spheroidal graphite, an incoculating alloy is added to the melt shortly before solidification so as to add 0.1 -0.2% by mass of Si to the melt, and solidification is followed by heat treatment to obtain at least one of the said basic structures.

6. A method as claimed in Claim 5 in which the workpiece is heated from the cast state to the austenitic range, where it is held for 0.25 to 10 hours and then cooled.

7. A method as claimed in Claim 4 in which, in order to produce a martensite hardened and tempered structure without precipitating secondary graphite, the workpiece is cooled from the austenite range in air, oil or water and then tempered at a temperature between 400 and 720"C.

8. A method of producing a workpiece as claimed in Claim 4, the method being substantially as described herein with reference to the foregoing Example.