WO2022146335A2

WO2022146335A2 - Hard surface austempered materials

Info

Publication number: WO2022146335A2
Application number: PCT/TR2021/051340
Authority: WO
Inventors: Hasan HASIRCI
Original assignee: Gazi Universitesi
Priority date: 2020-12-29
Filing date: 2021-12-03
Publication date: 2022-07-07
Also published as: WO2022146335A3; TR202022199A2

Abstract

The present invention relates to austempered cast iron and steel materials with high surface hardness.

Description

HARD SURFACE AUSTEMPERED MATERIALS

Technical Field of the Invention:

State of the Art:

After austenitizing the materials such as cast iron or steel at temperatures of 850-1100 °C for 0.5-1.5 hours, austempered materials are produced by austempering heat treatment by martempering and tempering in a salt bath at 250-450 °C for 30-250 minutes. As a result of this heat treatment, it is observed that materials with an ausferritic or bainitic structure are produced. Another application form of this heat treatment is the two-step austempering heat treatment. As a result of this heat treatment, ausferritic or bainitic materials with similar structures and properties are produced.

In addition to the superior mechanical properties of the austempered materials, they are exposed to abrasion, especially in the working environment of these materials. Therefore, there is a need for materials with a harder surface. In order to improve antiabrasion properties thereof, Thermochemical processes such as surface hardening with induction, martempering, surface coating, boronizing, nitriding are also performed. However, the layers formed on the surface by surface coating and thermochemical means are either easily deteriorated and abrasion resistance decreases, under abrasion conditions or due to various reasons in the production conditions, the layer thicknesses are limited (1 -1500 pm) and are insufficient. Performing above-mentioned processes requires both costs and special conditions.

When it is observed that classical austempering was not sufficient especially in terms of hardness and strength, a two-step austempering process was developed. In the production method of this material, after austenitization, the material is cooled to a lower tempering temperature than the material process conditions, and it is then heated to a normal process temperature. The purpose of this process is to provide the formation of precipitate phases such as bainite and alloy carbide in the structure and to produce harder and more durable materials. However, since this cooling process cannot provide sufficient hard phase formation in the material, or in other words, martensite formation, the targeted mechanical properties cannot be obtained. In addition, it is not possible to produce materials that are hard on the outside and firm on the inside since this process produces a similar structure both inside and outside. In some cast iron and steel materials, MC phases (alloy carbides) are also formed depending on the alloying elements. As a result of these phases, a sufficient increase cannot be achieved although the hardness of the material increases a bit more.

For the purpose of producing austempered material with a harder and abrasion resistant surface, after the austempering heat treatment, surface hardening is performed with different methods (flame, induction, thermomechanic, etc.) and materials with hard surface can be produced. However, in the production method of this material, the materials are first subjected to austempering heat treatment. Then, the surfaces of the austempered materials are somehow heated and then martempered. This process has some drawbacks. These are, since additional heat treatment is required, increase both in time and cost, structural deterioration or negativities (perlitic intermediate stage, which should not be between outer and inner microstructures, coarsening phases in austempered structure, residual austenite, untransformed austenite, etc.) in the austempered structure during the heating of the surface and after martempering, and the formation of inhomogeneities before and after the surface hardening process in parts with thickness differences or complex shapes.

On the other hand, there is also heat treatment between the surface and the center thereof, where the same high hardness value can be obtained. Iron-based materials with high hardness properties are produced by martempering heat treatment. This process consists of first austenitizing under suitable processing conditions, then martempering (in water, oil, air, etc.) and finally tempering. As a result of the martempering process, a martensitic structure is produced in both the surface and internal structure of iron alloys. Therefore, the material with high hardness due to the martensitic structure is also brittle. Because the martensitic structure reduces the toughness. The working life of these heat-treated materials is shortened as cracking and breaking occur due to high stresses. Desired hardness values cannot be achieved in terms of abrasion resistance on the surfaces of austempered materials. Because as a result of austempering, 25-45 HRc hardness can be obtained depending on the chemical composition of the material and process conditions, while 40-65 HRc hardness can be obtained as a result of martempering. Therefore, either coating is made using different methods or secondary surface hardening heat treatments have to be applied.

Secondary surface hardening heat treatment is required, especially in austempered materials that need to have a hard surface. As a result of the coating process, a hard surface is obtained in the form of a very thin layer (0.1 -1 mm). This layer is generally insufficient. Obtaining a thick (1 -10 mm) layer is only possible with surface hardening processes. Therefore, there is a need to produce austempered material with a thick hard layer.

Austempered materials are preferred due to their superior properties. However, the surface hardness of these materials is insufficient. There is always a need for austempered materials and a material with high toughness with a hard surface, which is a combination of martempered materials with much higher hardness. However, under current conditions such materials can be produced in two or more processes.

In the state of the art, the application numbered “TR2012/15171” was reviewed. In the invention that is the subject of the application it is stated that "The invention relates to a steel quenching and hardening system that includes a liquid (oil) filled tank in which the material to be hardened is positioned, a pump motor that provides liquid transfer into the said tank by means of transport pipes, characterized in that it comprises a speed controller connected to the motor without the need of using a butterfly valve between said pump and the tank.”

In the state of the art, the application numbered “US5246510A” was reviewed. In the invention that is the subject of the application, a method for producing a selectively surface hardened cast iron part includes the steps of heating the part to a desired austempering temperature of between about 450 °F and about 800 °F until the entire cast iron part possesses the desired austempering temperature substantially uniformly throughout it. The method comprises producing a cast iron part that has a selectiveiy surface-hardened, and the interior of the cast iron part remains substantially unhardened and unheated above the austempering temperature.

In the state of the art, the application numbered “KR2004057427A” was reviewed. The invention that is the subject of the application comprises an austempered heat treatment method that can shorten the heat treatment time and accordingly, reduce energy consumption by partially austempering only a desired part of the work by heat treatment. The method comprises shortening the heat treatment time by allowing the austempering heat treatment to be carried out locally only on a desired portion, thereby consuming energy.

Austempered materials are preferred due to their superior properties in studies carried out in the state of the art. However, the surface hardness of these materials is insufficient. There are no materials that have both high mechanical properties (tensile strength, toughness, ductility and hardness, etc.) and high abrasion resistant. Austempered materials with hard surface, the surfaces of which has martempered properties and the internal structure of which has austempered properties are required.

Consequently, the disadvantages disclosed above and the inadequacy of available solutions in this regard necessitated making an improvement in the relevant technical field.

Objects of the Invention:

The most important object of the present invention is to provide a material the outer part of which is martempered and the internal structure of which is austempered.

Another important object of the present invention is to provide a material having a hard surface and the internal structure of which has high toughness.

Another object of the present invention is to provide much higher ductility and toughness values while obtaining close hardness and strength values in the material. Structural and characteristic features of the present invention as well as all advantages thereof will be understood more clearly from figures disclosed below and the detailed description written by making references to these figures. Therefore, the assessment should be made by taking these figures and the detailed description into consideration.

Description of the Figures:

Figure -1 is the drawing that illustrates a graphical view comparing the mechanical properties of the material according to the present invention.

Figure ~2 is the drawing that illustrates operation temperature and time graphic of the process steps of the method according to the present invention.

Figure -3 is the drawing that illustrates the microscopic view of the hard surfaces and the internal structure with high toughness of the cast irons material produced according to the present invention.

Figure -4 is the drawing that illustrates the microscopic view of the hard outer part and the inner part with high toughness of the steels material produced according to the present invention.

Description of the Invention

Austempered materials are preferred due to their superior properties. However, the surface hardness of these materials is insufficient. There is always a need for austempered materials and a material with high toughness with a hard surface, which is a combination of martempered materials with much higher hardness. However, under current conditions such materials can be produced in two or more processes. There are no iron and steel materials, the surfaces of which is hard austempered with a single heat treatment without requiring additional processes and costs.

Iron alloys (cast iron and steels) are the most used materials in the production of complex shaped and 3-dimensional parts (except wire, bar, profile, plate, sheet). Complex shaped and 3-dimensional parts are usually shaped by means of using the casting method. Forging, rolling, and extrusion processes cannot be applied after casting. Various heat treatments are generally applied to these parts in order to improve their properties after casting. In the literature data, cast Irons have the lowest average properties, while the highest strength and hardness are obtained in martempered materials due to the martensitic matrix. However, it is observed that the ductility and toughness values of the martempered material are quite low. In the hard surface austempered material of the present invention, close hardness and strength values are obtained, while much higher ductility and toughness values are provided.

Production method of the hard surface austempered materials comprises the following process steps:

- First of all, austenitizing the material at 850-900°C for 30-90 minutes,

As a result of austenitization, the ferritic or ferrite + pearl itic or pearlitic structure in cast irons and steels turns into an austenite matrix structure.

- Placing it in the rapid cooling environment (water, salt water, ice water, oil, etc.) for a short time (1 -10 seconds depending on the desired hard layer thickness) for only the material surface to harden, after austenitizing,

Thus, the outer structure (sufaces) of the material have a hard surface since they are martensitic or martensite + carbide, while the internal structure (interior) have an untransformed austenite + ausferritic / bainitic structure, which causes higher toughness.

- tempering at 250-450°C for 30-240 minutes (130) after being placed in a rapid cooling environment (120).

As a result of these heat treatment steps, the surfaces of the materials is harder and more durable, and the internal structure is more tough and ductile, both abrasion resistant and impact resistant. This method ensures that the properties that can be obtained from two heat treatments are obtained in a single process by only quenching the material surface with the applied process. Materials in two different characteristics that can be obtained from two step heat treatments are obtained and thereby producing functional material. With the method applied to the iron and steel, “Hard Surface Austempered Spheroidal Graphite Cast Irons”, “Hard Surface Austempered Vermicular Graphite Cast Irons”, “Hard Surface Austempered Lamelar Graphite Cast Iron”, “Hard Surface Austempered White Cast Iron", “Hard Surface Austempered Plain Carbon Steels” and “Hard Surface Austempered Alloy Steels” can be produced.

In Figure 1 , the comparison of different materials with iron materials with hard surface in terms of some mechanical properties was given. If the austenitized materials are tempered at a low temperature (<350 °C), a harder and more durable material with a lower ausferritic or bainitic structure can be produced, if they tempered at high temperature (>350 °C), a more tough and ductile material with upper ausferritic or bainitic structure can be produced. Both situations have their own faults. By means of the material according to present invention, especially in parts where austempered materials are used intensively, such as gears, cranks, cams, carrier elements, excavator and crusher elements, etc., materials with both sufficient strength and hardness against abrasion, resistance against impacts and sufficient ductility can be produced.

Claims

CLAIMS 1. Austempered materials with high surface hardness, characterized in that, it has a hard surface since the surfaces are martensitic or martensitic carbide, and the internal structure have an ausferritic / bainitic structure, which causes higher toughness. 2. Austempered materials with high surface hardness according to Claim 1 , characterized in that, it is made of “Hard Surface Austempered Spheroidal Graphite Cast Irons”, “Hard Surface Austempered Vermicular Graphite Cast Irons”, “Hard Surface Austempered Lamelar Graphite Cast Iron”, “Hard Surface Austempered White Cast Iron”, “Hard Surface Austempered Plain Carbon Steels” and “Hard Surface Austempered Alloy Steels”. 3. Austempered materials with high surface hardness according to Claim 1 , characterized in that, a layer with higher surface hardness than internal structure are obtained by using element instead of water and performing austenitization. 4. Austempered materials with high surface hardness according to Claim 3, characterized in that, the elements used in austenitization can be carbon, nitrogen and boron. 5. Austempered materials with high surface hardness according to Claim 1 , characterized in that, the surfaces is hard and strength compared to the internal structure, the internal structure is tough and ductile compared to the surfaces in order ensure resistance to abrasion and impacts.