WO2002053793A1

WO2002053793A1 - Duplex process of diffusion forming of hard carbide layers on metallic materials

Info

Publication number: WO2002053793A1
Application number: PCT/HR2001/000053
Authority: WO
Inventors: Mladen Stupnisek; Bozidar Matijevic
Original assignee: Mladen Stupnisek; Bozidar Matijevic
Priority date: 2000-12-29
Filing date: 2001-12-20
Publication date: 2002-07-11
Also published as: HRP20000916B1; HRP20000916A2

Abstract

Duplex process of diffusion forming of hard carbide surface layers on the metallic materials contains two diffusion processes: diffusion carburising and diffusion forming of carbide layers. Significantly higher amount of carbide than required for forming carbide layer in the subsequent step of process is introduced by previous carburising. This contributes to forming of two hard surface layers (carbide layer on the substrate surface and case hardened martensite layer in the sub-surface zone of metallic material), Figure 3. Previous carburising prevents decarburization beneath the carbide layer, and contributes to the hardness beneath the carbide layer. Thus, high wear resistance of the surface is achieved, and service life of machine parts and tools for processing of metallic or non-metallic materials is improved. Carburising is conducted in solid, liquid, gaseous, ionized or fluidized agents in the temperature range 850-1100 °C, and diffusion forming of carbide layers in solid, liquid, gaseous or fluidized agents containing carbide forming elements Cr, V, Nb, W, Mo, Ti or Ta in temperature range 850-1200 °C.

Description

DUPLEX PROCESS OF DIFFUSION FORMING OF HARD CARBIDE LAYERS

ON METALLIC MATERIALS Technical Field Subject of the invention is a process of diffusion forming of hard surface layers (carbide and case hardened layers) on metallic surfaces, intended to improve the wear resistance. According the International Patent Classification, invention is classified into C23C8/00 - "Solid state diffusion of non-metal elements into metallic material surfaces" group and C23C10/00 - "Solid state diffusion of only metal elements or silicon into metallic material surfaces" group.

Invention is upgrading the application of diffusion forming of hard surface layers, since the use of previous carburising enables forming of hard case hardened martensite inter-layer, thus contributing to thicker carbide layers and providing harder base for carbide layer. Thus resistance towards the tribolological wear of treated structural parts and tools is improved, which in turn improves service life and quality. Background Art

Processes of diffusion forming of carbide layers are applied for machine parts and tools, which are made of ferrous materials, containing higher content of carbon required for forming of carbides on the surface. If more carbon in the base material is available, carbide layer forming rate will be higher. Accordingly, materials containing higher contents of carbon, i.e. those with higher carbon activity, are more suitable for such processes. Since the large quantity of carbon from base material is consumed in the process of diffusion forming of carbide layer, which contains approximately 17% of carbon, sub-surface layer of base material is partially decarburised. The consequence is reduced hardness of sub- surface layer. Thus, also mechanical and tribological resistance of surface of machine parts and tools are reduced. This phenomenon occurs at all diffusion process of carbide layers forming, independently on the type of carbide-forming element (V, Cr, Nb, W, Ti,..), agent (solid, liquid, gaseous, fluidized) or composition (metals, fero-alloys, oxides, reducents) as specified in patents DE 2819856 C2, DE 3025033 Al, EP D 161 684 Bl and other. Basic characteristic of these processes is that carbon from substrate (steels and alloys) participates in forming of carbide layer on the surface, and consequently, concentration of carbon in the sub-surface layer beneath the carbide layer is reduced. Hardness of such decarburised part of substrate is lower, and hard surface carbide layer is supported by significantly softer structure. The existing processes are not solving problem of such decarburization. Process registered by patent application GB 2 204 327 A is offering partial improvement, but time consuming, multiple repeating of process is required. Process does not eliminate fully the occurrence of decarburization beneath the carbide layer; it just contributes to forming of carbide layer. This process is conducted at lower temperatures (560 - 720 °C) when reaction rate is low, and in the ferrite phase, which has low solubility for carbon. Process registered by patent application JP 58174567 A is simpler in technical performance than one mentioned earlier, but again, problem of decarburised layer has not been solved. It merely effects the conditions of carbide layers forming. Besides, process is limited only for a partial application using only one kind of agent (paste made of powdered charcoal and activators LiCO₃ and BaCO₃), only one process of subsequent formation of carbide layer (Ti, N, Νb, Ta or Cr and its oxides), and for extremely low carbon materials. Disclosure of the Invention The basic concept of the invention is to apply the process consisting of two connected steps: carburising (Figure 1) and diffusion forming of carbide layer (Figure 2) during which part of carbon introduced by carburising is used. The first step produces carburising of surface layer of metallic material at temperatures above 850 °C, when metallic materials have austenitic phase, and characterized by high solubility for carbon. Carburising is conducted by introducing essentially higher amount of carbon than required for subsequent forming of carbide layer during second step (Figure 3). Carburised layer in such manner contains sufficient amount of carbon for formation of carbide layer during the second step, and remaining carbon is used for a forming of case hardened martensite layer beneath the carbide layer that is achieved by the subsequent quenching. By such treatment two hard surface layers are obtained; so called casq hardened layer in the substrate and carbide layer on the substrate surface. Thus, high resistance of surface to the wear is achieved. Moreover, such process eliminates occurrence of decarburised layer, which is usual in other classic processes of diffusion formation of carbide layer in which previous carburising is not conducted. Advantages of such technical solution, compared to the existing ones, are:

1. The developed process provides additional hard intermediate layer (martensite in the so called case hardened layer), not just a carbide layer on the surface of metallic material, as it is a case in the patent registrations GB 2 204 327 A and JP 58174567A. . Carburization is performed once at temperature above 850 °C, when austenite phase has high solubility for carbon and when high diffusion rate is achieved, as opposed to the registered patent GB 2 204 327 A where it is performed several times (6...7) at lower temperatures (560...720 °C), when carbon solubility in ferrite phase is very low and diffusion rate is low.

3. Carburising may be done by any known process, independently of the type and composition of the medium (solid, liquid, gaseous, ionized or fluidized phase). It is essential, that carbon potential in the agent is higher than in the treated material. Existing solutions are related to carburising in the liquid hydrocarbon agent (GB 2

204 327 A) or proprietary paste of specified content (JP 581 174567 A) and used to create just one layer (carbide).

4. Diffusion forming of carbide layer may be done by any known process, independently of the type and composition of the medium (solid, liquid, gaseous, ionized or fluidized phase), for one of more carbide-forming elements simultaneously.

5. Process may be applied for materials having low or medium content of carbon, not just for the materials extremely low on carbon as specified in the process registered in patent JP 581174567 A. 6. Second step of the process, diffusion forming of carbide layer, is conducted immediately after the carburising (Figure 4a), thus eliminating need for intermediate cooling and subsequent heating and making the procedure more cost effective. However, separate conduct of both steps is also possible (Fig. 4b). In such case, after carburising, treated parts are cooled down to the room temperature and than heated to the temperature of the second step.

7. Process may be applied also if only carbide layer is to be achieved at the surface of the metallic material, without additional intermediate layer but also without occurrence of decarburization during the carbide layer forming (Figure 2).

Figure 1: Diagram plots concentration of carbon in the surface of steel after the first step of duplex process (carburising) has been made. Amount of carbon introduced by carburising meets tlie requirements to produce carbide layer in the second step of duplex process

(diffusion forming of carbide layer on the steel surface).

Figure 2: Diagram shows changes in carbon concentration in a surface layer of steel during second step of duplex process (diffusion forming of carbide layer). Concentration of carbon is decreasing from the amount achieved after the carburization to the amount approximately equals those before the carburising.

Figure 3 Diagram shows changes in carbon concentration in a surface layer of steel during second step of duplex process, but in the case when significantly higher amount of carbon than required for forming carbide layer has been introduced. The surplus is intended for forming of hard intermediate layer in the substrate, beneath the carbide layer, so called duplex layer (case hardened martensite layer and carbide layer). This is particularly useful for low or medium carbon steels and alloys. Figure 4: Temperature-time diagram of the duplex process which consists of carburising and diffusion forming of carbide layer, which may be conducted directly one after another

(Figure 4a) or intermediate cooling may be applied (Figure 4b).

Figure 5: Cross-section of the carbide layer of Ck 45 steel grade produced without previous carburising. Figure 6: Cross-section of the carbide layer of Ck 45 steel grade produced applying previous carburising.

Figure 7: Hardness diagram for Ck45 steel grade, location beneath the carbide layer, common process without previous carburising.

Figure 8: Hardness diagram for Ck45 steel grade, location beneath the carbide layer after duplex process applaying carburising and diffusion forming of carbide layer conducted.

Figure 9: Cross-section of carbide and martensite layer of 20MnCr steel grade subjected to duplex process (carburising for 150 min in KG 6 granulate at 950 °C, diffusion vanadizing for 240 min at 950 °C, quenched in oil).

Figure 10: Hardness diagram of carbide and martensite layer of 20MnCr steel grade subjected to duplex process (carburising for 150 min in KG 6 granulate at 950 °C, diffusion vanadizing for 240 min at 950 °C, quenched in oil).

Essential concept or the invention is to set up a duplex process consisting of two steps. In the first step, amount of carbon required for formation of carbide layer (Figure 1) during the second step (diffusion forming of carbide layer-Figure 2) is introduced by diffusion. During this process, previously introduced carbon is used up, and metallic substrate contains approximately starting concentration of carbon of the treated metallic material.

Thus, occurrence of local decarburization is eliminated, the phenomenon common to conventional processes of diffusion forming of carbide layers without previous carburising.

In the other variation of the duplex process, significantly higher amount of carbon than required for subsequent formation of carbide layer is introduced by carburising (Figure 3). During the second step only the portion of the additionally introduced carbon is used for forming of carbide layer, thus leaving significant carburised layer which produces so called case hardened martensite layer by subsequent quenching. Hardness of this layer is much higher than the hardness of core of the quenched metallic substrate. Hard martensite layer is a good support for even harder carbide layer, thus contributing to higher wear resistance. Such duplex process produces duplex layer consisting of carburised martensite and on its top even harder carbide layer. Beside the advantageous effect of previous carburising upon achieving hard intermediate layer, such carburising contributes to the considerably higher forming rate of the carbide layer. Thus costs of treatment are also reduced. 1^st example of the patent conduct:

Sample of Ck 45 steel grade steel has been carburised at 950 °C in the Durferrit KG 6 granulate for 30 minutes, and then directly transferred into the salt bath for diffusion forming of carbide layer. There it was held for 240 minutes at 1000 °C. After this period of time, direct quenching in the water was done. For comparison, another sample of the same steel was treated in the same procedure in the same bath, but without previous carburising. Both samples were cut and cross-sections were ground and polished to provide conditions for micrographic analysis and micro-hardness testing. Thickness of carbide layer without previous carburising was 4,4 μm (Figure 5), and in the sample that was carburized in the preceding step thickness of the carbide layer was 6,5 μm (Figure 6). Hardness of the surface layer not subjected to previous carburising was less than 600 HN (Figure 1), while in the previously carburised steel it was in excess of 700 HV (Figure 8). 2^nd example of the patent conduct: In the first step, samples of 20MnCr5 steel grade were carburised in the Durferrit KG 6 granulate at 950 °C for 150 minutes. In the second step, samples were submerged into the salt bath for diffusion vanadizing at 950 °C, held there for 240 minutes and directly quenched in oil. In the transversal cross-section of sample, measured thickness of the carbide layer was 6 μm (Figure 9), while the increased hardness was registered in the martensite layer approximately 1500 μm thick (Figure 10).

Process may be applied in the metal and non-metal engineering industry, fabrication of machinery and devices, primarily in fabrication of tooling for metal forming, casting of light and non-ferrous alloys (intake nozzles, jokes, dies), processing of metallic and ceramic powders (dies,), processing of polymer materials (extruders, nozzles, dies) and machine parts for pumps for transportation of abrasive media. Hard carbide layers contribute to the wear resistance, thus improving service life of machine parts and tools and cost effective performance. Previous carburising may be executed by any method, conventionally used in industry, aimed to produce case hardened structural parts of a machine (cogwheels, shafts). Such processes are those in which carburising is made within granulate (e.g. Durferrit KG), salt baths (e.g. Cecontrol-Degussa), gas atmosphere (ENDO atmospheres, Supercarb, Carbomaag), ionized atmospheres and fluidized baths. Diffusion forming of carbide layers may be conducted by any process (Toyota Diffusion Process, Degussa,...) in the solid, liquid, gaseous or fluidized agent for all carbide forming elements (V, Cr, Nb, W, Ti, Ta, ..).

Claims

1. Duplex process of diffusion forming of carbide layers in the metallic materials that can dissolve carbon, characterized by, the feature of consisting of process of carburising of metallic material in the agent having carbon potential higher than in the treated material and process of diffusion formation of carbide layer upon the treated material in the agent containing carbide forming elements and additions required to improve carbon-carbide forming elements reaction.

2. Process according to the claim 1, characterized by, the fact that diffusion forming of carbide layer is performed without significant cooling of treated material in the short time after termination of carburization, or diffusion forming of the carbide layer is conducted after the cooling of the treated material to the ambient temperature and reheating to the temperature of diffusion forming of carbide layer.

3. Process according to the claims 1 and 2, characterized by, the fact that carburising is conducted in solid, liquid, gaseous, ionized or fluidized agents that have carbon potential higher than in the treated material.

4. Process according to the claims 1 and 2, characterized by, the fact that carburising is performed in temperature range of 850...1100 °C, when metallic materials are of austenitic phase, which posses high solubility for carbon.

5. Process according to the claims 1 and 2, characterized by, the fact that additional amount of carbon required for formation of carbide layer is introduced by carburising.

6. Process according to the claims 1 and 2, characterized by, the fact that amount of carbon introduced by carburising is significantly higher than that required for forming of carbide layer, thus providing conditions for producing hard martensitic inter-layer beneath the carbide layer.

7. Process according to the claims 1 and 2, characterized by, the fact that diffusion forming of carbide layer is conducted in solid, liquid, gaseous or fluidized agent, containing one or more carbide forming elements: Cr, N, Νb, W, Ti or Ta either in elementary form or as ferro-alloy, or oxide or reactive addition (chloride, halogenide, fluoride, boride,..). These additions enable reaction between carbon and the carbide forming elements to produce carbide layer on the surface of metallic material.

8. Process according to the claims 1 and 2, characterized by, the fact that temperature of diffusion formation of carbide layer in temperature range of 850 -1200 °C.

. Process according to the claims 1 and 2, characterized by, the fact that process is applicable for iron, nickel, cobalt and their alloys, particularly for the steel grades with low and moderate content of carbon.

10. Process according to the claims 1 and 2, characterized by, the fact that after diffusion forming of carbide layer, quenching (direct or indirect) is made to obtain high hardness of the carburised layer beneath the carbide layer.