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(11) | EP 0 002 555 A1 |
| (12) | EUROPEAN PATENT APPLICATION |
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| (54) | Process of heat-treatment of articles of cast-iron containing nickel and chromium |
| (57) Articles, particularly wear-proof parts, of cast-iron alloyed with nickel and more
than 2% chromium is heat- treated in two phases, in order to improve wear-resistance.
In the first place the article is maintained at a temperature of 550 ± 75'C for two
to six hours, after reaching that temperature throughout, and in the second place
is maintained at a temperature of 775 ± 75°C for two to twelve hours, after reaching
that temperature throughout. Thereafter the article is left to cool. As a result of
this treatment, the chromium carbides are finely dispersed in the matrix. |
[0001] The invention relates to a process of heat-treatment of an article of cast-iron containing, as alloying components, at least nickel and chromium, in which the article is heated in two phases and allowed to cool, and to articles treated by the process. The invention is particularly, though not exclusively applicable to the treatment of so-called "wearproof" components.
[0002] An example of a nickel and chromium alloy of cast-iron is "Nihard 4", which conforms to the specification for "Ni-hard iron type 4" of International Nickel. Components made of such a material have an appropriate combination of toughness and hardness and are used in industry for various purposes, e.g. wear plates. They are cheap compared with either wear tiles made from aluminium oxide or non-alloyed steel wear plates having a welded wearproof layer(welded for example by "bulk-welding").
[0003] It has however appeared that excessive and uneven wear can occur in use. This must be ascribed, as investigation has shown, to the existence of coarse primary chromium carbides; the size of the chromium carbide grains depends on the heat-treatment applied.
[0004] A process of heat-treatment of cast-iron alloy articles is proposed in United Kingdom Patent Specification 342,334 (1930). The articles to be treated, after casting, are heated throughout to a temperature in the region of, and not exceeding 680°C to soften them, cooled, machined if necessary, hardened by heating throughout to a temperature of 780° to 820°C and cooled in air. The first heating stage is specifically described as being performed by heating the articles in a muffle or furnace having a temperature not exceeding 680°C for a sufficient period of time to ensure that they are heated throughout. Approximately three hours is said to have been found sufficient. Thus the articles are not maintained at 680 C for any length of time after reaching that temperature, and the same applies to the second heating stage. It is expressly stated that the treatment is not applicable to alloys having a Ni content of more than 6% or a Cr content of more than 2%.
[0005] The invention aims generally to provide,without substantial extra cost, an improved method of heat-treatment of chromium and nickel alloyed cast-iron wearproof components, in which wear resistance is increased, and particularly to provide such a process for cast-iron containing more than 2% chromium.
[0006] The process according to the invention is characterised in that the article contains more than 2% (by weight) of chromium, the first said phase consists of maintaining the article for a period of two to six hours at a temperature of 550 + 750C after heating it throughout to that temperature, and the second phase consists of maintaining the article for a period of two to twelve hours at a temperature of 775 + 75 C after heating it throughtout to that temperature, after which the article is left to cool.
[0007] The cooling step after the second heating phase may be carried out either in still air or in the furnace itself.
[0008] Preferably, the first phase of treatment is carried out at a temperature of about 500°C for about 4 hours after reaching this temperature and the second phase at a temperature of 750°C to 800°C for 4 to 8 hours after reaching this temperature.
[0009] The heat-treatment applied according to the invention results in change in the structure of the material such that a particularly fine dispersion of secondary chromium carbides occurs in the matrix, which results in considerably greater hardness. As far as is known this phenomenon has so far not been found in a "wearproof" cast iron alloy containing nickel and chromium (e.g. Nihard 4). This fine carbide dispersion provides a structure which is favourable as regards wear. Because of the fine chromium carbide dispersion occurring in the matrix, resistance to wear is increased and the breaking away of coarse carbides is prevented.
[0010] Embodiments of the invention will be described below by way of example with reference to the Figures-and in a few Examples.
[0011] Fig.l shows a graph in which the temperature T of an article during heat-treatment in a furnace is plotted as a function of the time t, to show limits for typical heat-treatments according to the invention; and
[0012] Photographs 1 and 2 show the structure of wear plates treated by a process embodying the invention, photograph 1 being at an enlargement of 50 x and photograph 2 at an enlargement of 200 x.
[0013] A few cast-iron wear plates, purchased from a foundry in Wetteren Belgium, were examined in order to investigate the effect of the heat-treatment. Two plates were marked A and B. These investigations related to:
a) chemical composition
b) microstructure and
c) hardness.
[0014] The investigations b and c were carried out both before and after heat-treatment according to the invention. Table 1 gives the chemical compositions.
[0015] The properties of such cast-iron alloys are dependent upon, inter alia,their composition; in other words, the hardness is attributed to the effect of the Cr, the ductility to the Ni and the fineness of the grains to the Mo.
[0016] As shown by Fig.l, the plates were heated up to a temperature of between 475 and 6250, on average about 550°C, and thereafter maintained at this temperature for 2 to 6 hours, on average about four hours. In Fig.l this phase is indicated by I. Next the temperature was increased to about 7500 + 750C and this temperature was then maintained for from two to twelve hours, on average for 6 hours (phase II). Finally the plates were cooled in still air to room temperature.
[0017] In Fig.l the minimum and maximum times for the heat-treatment between the maximum and minimum temperatures in both phases are represented by shading. Between the two phases the said component can remain in the furnace, with phase II immediately following phase I, as shown schematically by the top dashed line. Another possibility is that between the two phases the article is taken out of the furnace and cooled slowly in still air either to, or to about room temperature. This latter possibility is recommended if the furnace has to be available for other purposes. b) Microstructure: All the wear plates, as supplied, exhibited coarse chromium carbide dispersion in a bainitic matrix. After conclusion of the heat-treatment embodying the invention, a very fine dispersion of secondary chromium carbides exists in the matrix.
[0018] In photograph 1, which shows an enlargement by 50 x of the cross section near the surface of wear plate B, the primary and secondary chromium carbides are clearly visible. In photograph 2, showing an enlargement by 200 x of the cross section near the surface of wear plate A, are even more details of these carbides is visible.
[0019] c) Hardness: The hardness of the material as supplied was on average 54 HRc or 570 HV. After the heat-treatment embodying the invention the hardness of plates A and B proved to be considerably greater, namely 59 HRc or 700HV, and it was 62 HRc in a third plate C. This can be explained by the dispersion of the secondary chromium carbides in the matrix.
[0020] The resistance to wear of treated wear plates in a duration test in a charging hopper of a blast furnace proved to be greater than might be expected solely from the increase in hardness.
[0021] This greater durability is not so much the result of the intrinsic increase in hardness, but rather that, due to the heat-treatment according to the invention, the chromium carbides, which are present in a relatively large quantity, are finely dispersed in the matrix. These carbides increase the resistance of the material to abrasive wear to a greater extent than the increase in hardness alone would lead one to expect, for two reasons: first because of the high intrinsic hardness of these carbides themselves, and secondly, as a result of this, the coarser eutectic carbides will continue to be surrounded by material for longer during the wearing process, causing them to break away less readily. The reproducibility of the structure arrived at, in an alloy with the required chromium and nickel contents, has also been clearly proved.
[0022] Wear plates of different sizes are usually cut for manufacturing purposes using plasma-cutting; this process makes a number of small cracks perpendicular to the burned edge, which result from shrinkage after cutting. This shrinkage proved to be so excessive in the wear plates treated according to the invention that cracks occurred parallel to the surface of the cutting-edge as well, causing spontaneous fracture. Tests have shown that this fracture can be prevented by forced cooling by spraying a jet of water immediately behind the cutting head of the plasma-burner.
1. A process of heat-treatment of an article of cast-iron containing nickel and chromium,
in which the article is heated in two phases and allowed to cool, characterised in
that the article contains more than 2% (by weight) of chromium, the first said phase
consists of maintaining the article for a period of two to six hours at a temperature
of 550 + 750C after heating it throughout to that temperature, and the second phase consists of
maintaining the article for a period of two to twelve hours at a temperature of 775
+ 75°C after heating it throughout to that temperature, after which the article is
left to cool.
2. A process according to Claim 1, characterised in that the cooling after the second
heating phase is performed in still air.
3. A process according to Claim 1 or Claim 2, characterised in that the article is
submitted to an intermediate cooling treatment in still air between the two heating
phases.
4. A process according to any one of claims 1 to 3, characterised in that in the first
heating phase the article is maintained at a temperature of about 500 C for about 4 hours (after reaching that temperature) and in the second heating phase
is maintained at a temperature of 750°C to 800°C for 4 to 8 hours (after reaching
that temperature) .
5. A cast-iron article, e.g. a wear plate, obtained by a process of heat-treatment
according to any one of claims 1 to 4.