GB2079315A

GB2079315A - Ferritic spheroidal-graphite iron for casting thick sections without segregation

Info

Publication number: GB2079315A
Application number: GB8119841A
Authority: GB
Original assignee: Creusot Loire SA
Current assignee: Creusot Loire SA
Priority date: 1980-07-01
Filing date: 1981-06-26
Publication date: 1982-01-20
Also published as: FR2486100A1; DE3125986A1; BE889425A

Abstract

The casting contains, by weight: 3.40 to 3.70% total carbon, 4.10 to 4.40% equivalent carbon, 2.0 to 2.22% silicon, 0.20 to 0.30% nickel, 0.10 to 0.20% manganese, 0.05 to 0.07% magnesium, up to 0.04% copper, up to 0.05% chromium, less than 0.08% molybdenum, less than 0.10% vanadium, less than 0.002% lead, antimony and bismuth together, less than 0.015% titanium, less than 0.010% cerium, less than 0.010% sulphur, and less than 0.040% phosphorus, and has, as rough cast, a minimum dimension of from 100 to 230 mm together with the following characteristics, as tested on the material in the core of the metal mass, a proportion of ferrite of at least 90%, a tensile strength of at least 360 MPa, an elastic limit of at least 250 MPa, an elongation of at least 14%, and a Charpy impact strength (standardised to KCU) of at least 23 J/cm<2>.

Description

SPECIFICATION Cast iron castings containing spheroidal graphite The present invention relates to massive and thick cast iron castings containing spheroidal graphite and which have a ferritic structure and are roughcast.

Iron castings containing spheroidal graphite are generally obtained by incorporating a small amount of magnesium into a suitable grade of base iron; they have much better mechanical characteristics than iron castings containing lamellargraphite. Iron castings containing lamellar graphite are generally used with a perlitic matrix produced directly from castings and hence without structural heat treatment.

Cast iron containing spheroidal graphite can have a perlitic, ferritic or mixed perlitic/ferritic structure, which gives it a wide range of possible mechanical properties. Of these iron structures, ferritic iron containing spheroidal graphite has the highest values of elongation under tension and of impact strength.

This type of cast iron can be obtained either directly from casting, without heat treatment, or after a structural heat treatment. The latter converts the ferrite to ferrite and is carried out at a holding temperature of 950 or 750 C, depending on whether or not the iron, in the crude state, contains iron carbides which, if present, detract from the elongation at break under tension and the impact strength. The required holding time at these temperatures depends on the thickness of the casting, the thicker the casting, the longer the holding time. In bulky castings, the high temperatures and long holding times required are likely to lead to deformation of the casting, even with very carefully adjusted furnace settings.

Because of their slow cooling rate after casting, very thick iron castings containing spheroidal graphite also tend to exhibit segregations in the core of the casting. Such segregations reduce the mechanical characteristics of the casting and can lead to fractures during service.

We have now found that by using a particular composition of iron for casting, it is possible to obtain massive castings which have a ferritic structure without the need for any heat treatment for this purpose and which have a homogeneous structure, even in the core ofthethick parts. Furtherthe castings obtained have an advantageous combination of improved characteristics as regards elastic limit, tensile strength, elongation and impact strength.

According to the present invention, there is provided an iron casting which contains spheroidal graphite and has a ferritic structure and which contains by weight: 3.40 to 3.70% total carbon, 4.10 to 4.40% equivalent carbon, 2.0 to 2.22% silicon, 0.20 to 0.30% nickel, 0.10 to 0.20% manganese, 0.05 to 0.07% magnesium, up to 0.04% copper, up to 0.05% chromium, less than 0.08% molybdenum, less than 0.10% vanadium, less than 0.002% lead, antimony and bismuth together, less than 0.015% titanium, less than 0.010% cerium, less than 0.010% sulphur, and less than 0.040% phosphorus, the metal mass of the casting, as rough cast, having a minimum dimension of from 100 to 230 mm and the casting having the following characteristics, as tested on the material in the core of the metal mass, a proportion of ferrite of at least 90%, a tensile strength of at least 360 MPa, an elastic limit of at least 250 MPa, an elongation of at least 14%, and a Charpy impact strength (standardised to KCU) of at least 23 J/cm2.

It is to be understood that the minimum dimension referred to is not necessarily that of the casting itself, since the latter may be hollow, but refers to the minimum dimension of the metallic mass making up any part of the casting.

It is preferred that spheroidal graphite should constitute at least 80% of the total graphite present. The casting is preferably free of carbides or contains not more than a very small amount of carbides.

The castings according to the invention can be processed by known methods, but it is preferred to use a combination of preferred conditions and precautions, as follows: use of a large capacity low-frequency electric induction furnace which enables the chemical composition of the iron, the superheating of the iron melt and the casting temperature, which is preferably from 1300 and 1 330 C, to be precisely controlled; careful choice and control of the starting materials used to make the melt: the raw materiais are selected according to their appearance, their analysis and their purity level; careful choice and control of the nature and the order of introduction of the constituents of the charge; the melting rate; and the nature and the amount of final constituents added to the liquid iron before it leaves the furnace; through desulphurisation of the metal by mixing until the sulphur content is less than 0.010%; the addition of the magnesium content in the form of a ferro-silico-magnesium alloy containing about 15% of magnesium; and inoculation with a ferro-silicon containing about 75% of silicon, at the end of the magnesium treatment, while mixing the metal.

Afterthe inoculation treatment, the iron containing spheroidal graphite thus obtained is run into a mould in the form of the desired casting and is allowed to solidify.

The casting is not subsequently subjected to any high temperature structural heat treatment which, on bulky castings, may tend to cause deformations.

On the other hand, in certain cases, the casting can be subjected to a low temperature stress-relieving heat treatment, for example at about 550 .

The chemical analysis of the massive casting obtained conforms to the above-mentioned analysis range according to the invention. Preferably, the casting is free of carbide or contains only a very small amount of carbides and the proportion of spheroidal graphite is at least 80% of the total graphite.

As will be understood, the essentiai advantages of the massive castings according to the invention con sistofthe remarkable mechanical properties which have been mentioned above, these properties being evidenced even in the core of the thick parts, because the casting has a very homogeneous structure.

The avoidance of a high temperature structural heat treatment brings about several substantial advantages: no deformation likely to jeopardise the dimensions of the finished casting; no oxidation, which would necessitate cleaning of the castings by shot-blasting or sand-blasting; and no requirement for a heating furnace of large dimensions.

In order that the invention may be more fully understood, a particular casting according to the invention will now be described by way of example only and with reference to the accompanying drawings, in which: Figure lisa cross-section of the casting on a plane passing through its axis of revolution, Figure 2 is the section (on an enlarged scale) of a block a b c d taken from the wall ab, having a thickness of 230 mm, of the casting, from which block test pieces for testing impact strength and tensile strength are cut, and Figure 3 is the section (on an enlarged scale) of a block e f g h taken from the same casting, the test pieces being taken from a wall, having a thickness of 110mm,ata depth of55 mm.

The casting is a body of revolution having a weight of 15 tonnes, a height of 1240 mm, an external diameter at the base of4510 mm, and an internal diameter at its upper part of 3100 mm.

This casting is made of a grade of iron containing spheroidal graphite and having the following analysis, by weight: total carbon 3.45% equivalent carbon 4.12% silicon 2.02% sulphur 0.007% phosphorus 0.020% manganese 0.130% magnesium 0.053% nickel 0.27% The average ferrite content is 92% and the propor tion of spheroidal graphite is 80%.

At a b c d, a small block has been cut out, which is shown on an enlarged scale in Figure 2. In this Fig ure, the black circle represents the location of the test piece for testing tensile strength and the black square represents the location of the test pieces for testing impact strength.

For this thickness of 230 mm, and at 120 mm from the external wall of the casting, the three measurements of Charpy impact strength, standardised to KCU, were, in Joules per square centimetre: 26.3, 27.5 and 28.8, that is to say an average of: 27.5 J/cm2.

The average of the measurements on the three test pieces for testing tensile strength was found to be: 279 MPa for the elastic limit at 0.002 (that is to say 0.2%).

363 MPa for the tensile strength.

14.3% for the elongation.

Another block e f g h, shown on an enlarged scale in Figure 3, was cut from this same casting (ef = 210 mm). In this Figure also, the black circle represents the location of the test pieces for testing tensile strength and the black square represents the location of the test pieces for testing impact strength.

At this level, the thickness of the casting is 110 mm. For this thickness of 110 mm, and at about 55 mm from the external wall of the casting, the three measurements of Charpy impact strength, standardised to KCU, were, in Joules per square centimetre: 37.5,39.4 and 36.9, that is to say an average of: 37.9 J/cm2.

The average of the measurements carried out on the three test pieces for testing tensile strength was found to be: 252 MPa for the elastic limit at 0.002 (that is to say 0.2%).

387 MPa for the tensile strength.

25% for the elongation.

Claims

1. An iron casting which contains spheroidal graphite and has a ferritic structure and which contains, by weight:

3.40 to 3.70% total carbon,

4.10 to 4.40% equivalent carbon,

2.0 to 2.22% silicon, 0.20 to 0.30% nickel, 0.10 to 0.20% manganese, 0.05to 0.07% magnesium, up to 0.04% copper, up to 0.05% chromium, less than 0.08% molybdenum, less than 0.10pro vanadium, less than 0.002% lead, antimony and bismuth together, less than 0.015% titanium, less than 0.010% cerium, less than 0.010% sulphur, and less than 0.040% phosphorus, the metal mass of the casting, as rough cast, having a minimum dimension of from 100 to 230 mm and the casting having the following characteristics, as tested on the material in the core of the metal mass, a proportion of ferrite of at least 90%, a tensile strength of at least 360 MPa, an elastic limit of at least 250 MPa, an elongation of at least 14%, and a Charpy impact strength (standardised to KCU) of at least 23 J/cm2.

2. A casting according to claim 1, in which spheroidal graphite constitutes at least 80% of the total graphite present.

3. An iron casting substantially as herein described with reference to the accompanying drawings.