WO1979000274A1 - Manufacturing process of metal pieces made of alloy cast iron exposed to wear - Google Patents

Abstract

Hitherto metal pieces made of alloy cast iron Fe/Cr and exposed to wear, of which the basic mass had a martensitic character, contained, without any exception, secondary carbide deposits; this was due to the fact that these pieces, once cast, were first cooled to room temperature and, only after, submitted to high temperature treatment for the hardening. On the other hand, the metal pieces intended for exposure to wear and obtained according to the invention are substantially free of secondary carbide deposits; this is achieved as follows: the unmoulding of the moulded cores is done immediately after solidification and without any cooling under the perlite point AC<u1>u ; then, in order to destabilize the austenite, it is kept during five hours at least at a temperature above the perlite point AC<u1>u; after, the temperature is gradually lowered until it reaches room temperature, while avoiding, nevertheless, the formation of perlite and bainite. There is substantially no elimination of secondary carbides which cause deterioration of the physical characteristics of the pieces in question. The pieces obtained according to the invention are of improved and more uniform quality and have a higher resistance to deformation, while having the same resistance to wear.

Description

GEORG FISCHER AKTIENGESELLSCHAFT, Schaffhausen

(2030 / FES)

Process for the production of wear parts made of white solid alloyed chrome cast iron

The invention relates to a process for the production of wear parts from white-solid, alloyed chrome cast iron with a martensitic base material and to a product according to this process.

After cooling from the casting temperature to room temperature, white-solidified chrome cast iron alloys are predominantly solidified with numerous eutectic or primary carbides depending on the carbon and chromium content. By heating to temperatures between 900 and 1100 ° C for 0.5 to 8 hours, the austenite is destabilized by the precipitation of Cr, Mo, Fe and C in the form of secondary carbides, and when it is cooled again to room temperature, the austenite changes to martensite ¬ changed. With this conventional treatment, however, according to Diesburg and Borik (Symposium on Materials for the Mining Industry, Vail, Colorado, July 30/31, 1974) the secondary carbide precipitations cause the fracture toughness up to 35% and the flexural toughness (K __., _-) reduced by up to 40% without achieving an improvement in wear resistance.

_OMPI The invention has for its object to provide a method by means of which the toughness of wearing parts made of white solid, alloyed cast iron with a martensitic matrix can be increased in an economical manner without impairing the wear resistance.

According to the invention this is achieved by the characterizing features of the main claim. Precisely because secondary carbide deposits can be avoided, the toughness can be significantly increased. The wear resistance does not decrease.

Thanks to the energy savings for heating the room temperature to the destabilization temperature, which, depending on the alloy composition, lies between 160 and 200 kcal / kg cast iron, the wearing parts can be manufactured economically. It is no less important to reduce the environmental impact due to the heat radiation when the castings cool down in the production hall.

The destabilization temperature is preferably 900 to 1080 C, with a holding time of preferably at least 8 hours at this temperature. The molds used could be molds.

With this direct heat treatment according to the invention, the thermodynamic conditions for the precipitation of the secondary carbides are not met. The entropy of the white solid chromium cast iron alloys is essentially different when the hardness temperature is reached by cooling than by heating.No energetic or concentration peaks are formed as seed points for future secondary carbides in the austenite.The austenite is not so high with carbon and alloying element oversaturated and the segregations that may occur are largely compensated for by longer holding times at the hardening temperature. According to the invention

^• ^ UR

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Λ _* . WI The process supplies wear parts with a martensitic matrix or matrix, practically free of secondary carbide deposits.

With the method, wear parts with a volume fraction of secondary carbide precipitates of less than 1% are possible.

Particularly good (more uniform and improved quality and reduction of the risk of breakage) wearing parts have a C content of ^ 2.3% and a Cr content of * θ%. Further advantageous compositions result from claims 8 to 10.

The difference between conventional and inventive wearing parts is most striking when the electron microscopic images of the corresponding metallurgical sections are compared:

1 shows the numerous secondary carbide precipitates contained in the martensitic matrix of a conventionally heat-treated wearing part, which are practically missing in the wearing part according to the invention (FIG. 2).

Our own investigations show that the wearing parts according to the invention have a hardness of 60 to 65 HRc and a microhardness of the matrix of up to 1000 HVm (25 g) with a Cr content of 15 to 28%. The laboratory wear tests show that the wear parts according to the invention in the secondary carbide-free state have the same or more favorable wear resistance than conventionally heat-treated wear parts.

The following materials are particularly suitable chromium cast iron alloys:

_. OMPI ö % C% Cr% Ni% Mo% Cu

G-X 300 -CrNiSi 952 2.5-3.5 8-10 4.5-6.5 to 0.5

2.5-3.5 9-11 2 -3 to 1.0 to 3

G-X 300 CrMo 153 2.3-3.6 14-17 1-3

G-X 300 CrMo 1521 2.3-3.6 14-17 0.8-1.2 * 1.8-2.5

2,3-3,6 14-17 to 3 to 1

G-X CrMoNi 2021 2.3-3.0 18-22 0.8-1.2 * 1.4-2.0

G-X 260 Cr 27 2.3-3.0 23-28 to 1.2 to 1

G-X 300 CrMo 271 2.5-3.6 27-30 to 1.2 to 1

* Ni can be replaced by Cu. The invention is explained in more detail below on the basis of exemplary embodiments.

Example 1: Alloy 15-3 (DIN GX 300 CrMo 15 3) After solidification, the casting is removed from the casting mold at a unpacking temperature of 900 to 1150 C and charged directly into an oven preheated to the destabilizing temperature without cooling below the pearlite point A. . The temperature is equalized in the furnace and the casting is kept at this temperature for at least 10 h (practically unlimited upwards). The destabilization temperature is between 92 and 1000 C, depending on the C content and other alloy additives such as Cu, Ni etc. During the holding time, the austenite is homogenized by diffusion processes to such an extent that a martensitic transformation is possible. Controlled cooling after removal from the destabilization furnace converts the austenite to martensite. The cooling rate between the destabilization temperature and the bainitic nose B of the time-temperature conversion diagram shown in "FIG. 3 must be so great that the bainitic nose B (intermediate stage structure) is not touched. In the case of alloy 15-3, it corresponds to C 3 % approx. 25 C / min The semicolon cooling curve intersects the breakpoint lines A_ and A., passes through the austenite and carbide area A + C and runs away from the pearlite area F + C. The martensite starting line is identified by the reference symbol Mö.

After this heat treatment, a macro hardness of approx. 60 to 65 HRc and a micro hardness of the basic mass HVm (25 g) of approx. 700 to .50 are achieved. The proportion of residual austenite is approx. 10 to 20% for microscopic determination and approx. 35% for Khim diffractographic determination. This residual austenite content can be further reduced by tempering.

Example 2: Alloy DIN G-X 300 CrMo 27 1 After solidification, the casting is removed from the casting mold at a temperature of 800 to 1150 C and placed directly in an oven which is preheated to the destabilizing temperature of 1020 to 1080 C. The temperature is equalized in this furnace and the casting is kept at this temperature for at least 8 hours (no upper limit). During the holding period, the austenite is homogenized by the diffusion to such an extent that a martensitic transformation is possible. By controlled cooling, which must correspond to the desired structure, i.e. with a pearlite and bainite free structure of approx. 32 C / min., the austenite is converted into martensite without the elimination of secondary carbides.

The macro hardness reaches values of approximately 60 to 62 HRc, the microhardness of the matrix is approximately 700 to 900 HVm (25 g). After subsequent tempering, the macro hardness values for this alloy are 62 to 65 HRc and the micro hardness of the matrix between 800 and 950 HVm (25 g).

The residual austenite content after hardening is approximately the same as in Example 1.

Example 3: A DIN GX 300 CrMo 15 3 alloy was melted in an induction furnace (50 kg) and poured into a GG mold. Immediately after solidification - surface temperature approx. 1000 to 1100 C - the castings were used directly in an oven preheated to 960 C The individual samples were then removed after holding times of 1, 2, 3, 4, 5, 8, 12, 16, 24, 36 and 48 hours and cooled in still air.

For the wear test, two rods (0 6 mm and length approx. 30 mm) were machined from the test rod (0 40 mm with the aid of electroerosive machining) and subsequently ground.

In the test, this test stick rotates about its own axis (20 rpm) and at the same time moves back and forth over a fresh cloth with abrasive - garnet (calcium aluminum silicate) or Al _? 0_, among other things in certain grain size. In our case it was 100 um garnet. During the test, the test rod is loaded with a weight of 6.2 kg. The test takes 7 minutes, with a distance of 12.80 m in length. As a measure of the wear resistance di the weight loss of the sample. Each sample was tested twice. The best result in the wear test using the pin test was achieved after a holding time of 24 hours with a loss of 31 mg of wear. Wear tests with conventionally heat-treated similar alloys resulted in a loss of wear of 35 mg.

Claims

Patent claims 1. A process for the production of wear parts from white solidified, alloyed chrome cast iron with a martensitic base mass by means of a casting mold, characterized in that the casting is removed immediately after the casting mold has solidified, without cooling below the pearlite point A, for the purpose of destabilizing the austenite for at least five Hours at a temperature which is above the pearlite point A, and then cools as slowly as possible, but avoiding the formation of pearlite and bainite to room temperature. 2. The method according to claim 1, characterized in that the destabilization temperature is above 800 C. A method according to claim 2, characterized in that the stabilization temperature is between 900 and 1080 C. Method according to one of claims 1 to 3, characterized gekenn¬ characterized in that the holding time at the destabilization temperature is at least 8 hours. 5. The method according to claim 1, characterized in that a mold is selected as the casting mold. 6. Wear part according to claim 1, characterized in that the volume fraction of secondary carbide precipitates ^ 1% is. 7. wearing part according to claim 6, characterized by a content of. 2.3% C and ^ 30% Cr. 8. Wear part according to claim 7, characterized by the following alloy components: 2.5-3.5% C 8-11 _ Cr; 2-6.5% Ni; 3.5% Cu and 0.5-1% Mo. 9. Wear part according to claim 7, characterized by the following alloy components: 2.3-3.6% C; 14-17% Cr; 0.5-3% Ni and 0.5-3% Mo. 10. Wearing part according to claim 7, characterized by the following alloy components: 2.3-3.6% C; 18-30% Cr; 0.5-1.2% Ni and 1-2% Mo. -BUR _0M 'f - W1