TECHNICAL FIELD

The invention relates to an engine component comprising a corrosion- protection layer and to a method for manufacture of engine components which comprise a corrosion-protection layer.

BACKGROUND

Components of combustion engines are often subject to corrosion due to the conditions which prevail in the engine. A typical engine component is engine valves arranged in combustion engines to control the flow of air to the engine's cylinders and to control the flow of exhaust gases from the cylinders. During operation of the engine, the valves are subject to great stresses due to the high pressures and temperatures which prevail in the engine's cylinders. Particularly in diesel engines which use emission-reducing exhaust gas recirculation (EGR) for the engine's combustion, a raised content of nitric acid and sulphuric acid in the inlet air to the cylinders also occurs. In this context, the composition of inlet gases and exhaust gases often results in corrosion on valve discs and valve stems. The corrosion lowers the strength of the valve and increases the risk of failure, with serious consequences for the engine, particularly if the valve breaks in the valve disc. In engine valves, intercrystalline corrosion may occur in the valve disc, often leading to valve failure.

The valves are also subject to wear due to the repetitive movement which the valve performs in the engine. In particular, the valve stem is subject to wear caused by friction between valve stem and valve guide.

Examples of other components subject to corrosion in the engine are valve guides and valve seats. Coating of engine valves with protective layers is a known practice. Specification US5271823 refers to an engine valve which comprises a wear- resistant layer consisting of chrome oxide and chrome carbide. The layer is applied to the engine valve by electroplating of the valve in a liquid solution which contains chrome ions, followed by heat treatment.

Specification US4495907 refers to a method for coating of components in a combustion engine with wear-resistant and thermally insulating layers. According to the method, the component is first coated by flame-spraying with a layer of thermally insulating material. The layer formed is thereafter impregnated with a solution which contains a chrome compound. After completion of impregnation, the layer is heat-treated, thereby converting the chrome compound with which the layer is impregnated to chrome oxide.

The protection of engine valves with layers of metallic chrome is also known. These valves are usually coated by electroplating.

A problem with the layers described above is that they do not provide sufficiently good protection against corrosion of the substrate component. This is due largely to the fact that the layers applied do not adhere well to the surface of the part and do not exhibit sufficient tightness. The methods described are also labour-intensive and involve complicated thermal coating processes and handling of liquid chemicals. The layers formed by the above methods are also often thick, resulting in the components having to be ground to final dimensions in a subsequent grinding operation.

An object of the invention is therefore to propose an engine component which has high resistance to corrosion. A further object of the invention is to propose an effective method for manufacture of corrosion-resistant engine components. SUMMARY OF THE INVENTION

The above objects are achieved by an engine component which comprises a chromiferous steel and a corrosion-protection surface layer characterised in that the surface layer consists of iron chrome oxide.

The iron chrome oxide layer has high tightness and exhibits good adhesion to the engine component's surface. The engine component is thereby effectively protected against corrosion in a very acid environment. The thbological characteristics of the iron oxide layer also make it possible for it to be used as a wearing layer and to protect the stem of the valve against wear.

The engine component is with advantage manufactured by a method comprising the steps of:

- providing an engine component which comprises a chromiferous steel;

- heating the engine component at a predetermined temperature for a predetermined time so that a layer of iron chrome oxide is formed on the surface of the engine component;

- cooling the engine component to room temperature;

The iron chrome oxide layer is thus formed directly on the steel surface of the engine component by oxidation of chrome and iron on the surface of the engine component. The iron chrome oxide layer therefore acquires good adhesion to the steel surface of the engine component. The iron chrome oxide layer formed also has very good tightness which, in combination with the good adhesion, renders the layer very resistant to corrosion in an acid environment. The iron chrome oxide layer formed as above consequently protects the substrate component effectively against corrosion. The tribological characteristics of the iron chrome oxide layer also make it possible for it to be used as a wearing layer and to protect the component against wear. The engine component comprises with advantage a steel with a chrome content of at least 8 wt% and up to 88 wt% of iron. A chrome content of at least 8 wt% and iron are necessary for a layer of iron chrome oxide to be formed. The engine component preferably comprises steel with a chrome content of 20 - 22 wt% and an iron content of 58 - 65 wt%. The high chrome content promotes the formation of chrome iron oxides whereby a tight oxide layer is quickly formed upon heating of the steel.

With advantage, the engine component is heated at a temperature of between 150 ⁰ C and 500 ⁰ C. As the formation of the iron chrome oxide layer is a diffusion-controlled process, the temperature needs to be at least 150°C for the layer to grow. At lower temperatures, the growth of the layer ceases, rendering the layer too thin to provide good corrosion resistance. As the growth of the thickness of the oxide layer takes place very quickly at high temperatures, the temperature should not exceed 500 ⁰ C, because in that case the layer thickness would become difficult to control. At temperatures over 500°C there is also greater risk of changes in the structure of the steel and greater risk of deformation of the engine component's dimensions.

The period of time for which the engine component is heated depends on the temperature and the intended thickness of the layer. With advantage, the heating time adopted is within the range of 1 - 5 hours.

The engine component is preferably heated at a temperature of between 250 and 350°C for 1 - 3 hours. This results in a thin and tight iron chrome oxide layer which exhibits very good adhesion to the engine component's steel surface.

The layer consists with advantage of FeCr ₂ O ₄ and is of spinel type. Such an oxide is particularly suitable for corrosion protection in that it forms a tight layer largely free from pores. The layer has with advantage a thickness of 5 - 20 μm. The layer needs to be at least 5 μm thick to achieve good corrosion protection. High layer thicknesses increase the risk of the layer flaking. A layer thickness of up to 20 μm, preferably 10 μm, provides very good corrosion protection in combination with good wear resistance of the layer.

The layer has with advantage a hardness of about 1500 HVO.1. This results in good wear resistance of the oxide layer.

The engine component is preferably an engine valve or a valve guide or a valve seat. In modern highly stressed diesel engines with relatively large amounts of exhaust gas recirculation (EGR) these components are subject to corrosion in the acid environment. These components are particularly suited to protection by iron chrome oxide layers, being components usually made of chromiferous steel.

The invention relates also to a method for manufacture of an engine component comprising a corrosion-protection layer, characterised by the steps of:

- providing an engine component which comprises a chromiferous steel;

- heating the engine component at a predetermined temperature for a predetermined time so that a layer of iron chrome oxide is formed on the surface of the engine component;

- cooling the engine component to room temperature.

The protective layer is formed by oxidation of the engine component's surface without supply of substances other than oxygen from the atmosphere in the furnace. The result is a simple and effective method for manufacturing an engine component with a corrosion-protection layer. As the method makes it possible to form a thin and very tight layer, subsequent treatment such as grinding of the valve is avoided. According to an alternative, the engine component is heated in air. This results in a simple and cost-effective manufacturing method in that only air is supplied. A further advantage is that the method is suitable for simple types of furnace which are open to the atmosphere.

According to an alternative, the engine component is heated in air with raised oxygen content. The raised oxygen content causes the oxide layer to form more quickly, making it possible to minimise the heat treatment time.

In the method according to the invention, the engine component comprises with advantage a steel with a chrome content of at least 8 wt% and up to 88 wt% of iron, preferably a steel with a chrome content of 20 - 22 wt% and an iron content of 58 - 65 wt%.

The engine component is with advantage heated at a temperature of between 150 and 500 ⁰ C. The engine component is preferably heated at a temperature of between 250 and 350 ⁰ C for 1 - 3 hours.

DESCRIPTION OF THE DRAWINGS

Figure 1 : A side view of an engine component according to the invention.

Figure 2: A cross-section of an engine component according to the invention.

Figure 3: A flowchart illustrating the method according to the invention for manufacture of an engine component.

Figure 4: An enlargement of a sample from an engine valve which was heat-treated at 350°C in air for 3 hours. DESCRIPTION OF EMBODIMENTS

Figure 1 illustrates an engine component according to a first embodiment of the invention. It depicts an engine valve, but the engine component may also take the form of other components, e.g. a valve seat or a valve guide.

The engine valve 1 is intended to control the flow of air and exhaust gases in the cylinder of a combustion engine. With advantage, the engine valve is dimensioned for diesel engines to power heavy vehicles. These types of engines may use relatively large amounts of exhaust gas recirculation (EGR), particularly if this exhaust cleaning method is mainly intended to keep the engine's emissions below permissible limit values. Nevertheless, the engine valve may also be dimensioned for other types of engines, e.g. petrol engines for passenger cars or motor cycles.

The engine valve 1 comprises a valve disc 2 intended to cooperate with a valve seat in the engine. The valve disc 2 has in the diagram a planar upper surface 3 which, in the engine, faces towards the combustion chamber. The engine valve also comprises a valve stem 4 intended to move in a valve guide in the engine.

Figure 2 depicts a cross-section through the engine valve in Figure 1. The engine valve's body 6 is made of a chromiferous steel. The engine valve's surface or parts of its surface take the form of a layer 5 of iron chrome oxide. The iron chrome oxide layer is a spinel oxide with the chemical formula FeCr ₂ O ₄ and may be 5 - 20 μm thick. The layer is preferably 5 - 10 μm thick. The iron chrome oxide layer has a hardness of about 1500 Vickers (HVO.1 ) and is tight, i.e. without pores.

The method according to the invention for manufacturing an engine component which comprises a corrosion-protection and wear-resistant surface layer is described below. The main steps of the method may be followed in the flowchart in Figure 3. As a first step 100, an engine component made of a chromiferous steel is produced.

The engine component, e.g. an engine valve, is with advantage manufactured by forging and by cutting machining. The material in the engine component consists of a steel with at least 8 wt% of chrome and the remainder iron. It is important that the steel contains iron and at least 8 wt% of chrome if iron chrome oxide is to be formed during the subsequent heating. For example, the engine valve consists of a steel which contains 8 - 10 wt% of chrome and up to 88 wt% of iron, e.g. 86 - 88 wt% of iron. The remainder takes the form of other alloy substances, e.g. C, Si, Mn and Ni. The engine valve preferably comprises 20 - 22 wt% of chrome and 58 - 65 wt% of iron. The remainder takes the form of other alloy substances, e.g. C, Si, Mn, Ni, N, W, Nb and Ta. Examples of suitable steel grades are DIN 1.4718 and DIN 1.4822.

In a second step 200, the engine component is heated at a predetermined temperature for a predetermined time so that a layer of iron chrome oxide is formed on the engine component's surface.

To this end, the engine component is placed in a furnace and heated to the predetermined temperature. When the material heats up, the oxygen in the furnace atmosphere reacts with chrome and iron on the engine component's steel surface to form a tight layer of iron chrome oxide. As the oxide forms directly on the engine component's surface, the layer has very good adhesion to the surface.

The thickness of the layer increases thereafter as oxygen atoms from the furnace atmosphere diffuse, through the iron chrome oxide layer formed, to the underlying steel surface and oxidise the latter. The layer formed thus grows inwards from the surface of the engine component towards the centre of the engine component. The rate at which the oxygen atoms diffuse through the oxide layer formed depends on the temperature. High temperature results in a high diffusion rate, increasing the thickness of the layer quickly. At lower temperatures, the diffusion rate is lower and the layer therefore grows more slowly. The final thickness of the layer therefore depends on the temperature at which the layer is heated and how long the layer is heated.

The temperature adopted is with advantage in the range 150 to 500 ⁰ C. A temperature over 150 ⁰ C is necessary for the oxide layer to form. Over 500°C, the rate of growth of the oxide layer becomes too high, as the diffusion rate increases exponentially with temperature. The thickness of the layer therefore becomes difficult to control, increasing the risk of the layer becoming too thick. There is also greater risk of changes in the structure of the material and greater risk of deformation at temperatures over 500 ⁰ C.

The period of time adopted for heating the engine component is based on the intended thickness of the layer and the temperature at which the engine component is heated. With advantage, the time adopted is within the range 1 - 5 hours. The engine component is preferably heated at a temperature within the range 250°C - 350°C for 1 to 3 hours. This results in an iron chrome oxide layer with a thickness of 5 - 10 μm which is also tight and has good adhesion to the substrate. According to an particularly preferred embodiment, the engine component is heated at 350 ⁰ C for 3 hours, resulting in a 10 μm thick iron chrome oxide layer.

The furnace is heated electrically or by burners and may for example be a batch furnace for batch production or a pusher furnace for continuous production. The atmosphere in the furnace consists typically of air. According to an alternative, the atmosphere in the furnace may consist of air with raised oxygen content. Adding oxygen gas to the furnace atmosphere increases the growth rate of the iron chrome oxide layer in that more oxygen atoms are available for the oxidation process.

In a further step 300, the engine valves are cooled to room temperature. According to an alternative, the cooling is effected by the engine valves being taken out of the furnace and being placed in still air until they have cooled to room temperature. According to a further alternative, the engine valves are cooled by a fan.

DESCRIPTION OF EXAMPLE

A concrete example illustrating the invention in more detail is described below.

Four engine valves are made of a steel of grade DIN 1.4822. The valves are numbered MV1 , MV2, MV3, MV4. Samples are sawn from the disc of each valve MV1 , MV2, MV3, MV4. The samples sawn from valves MV3 and MV4 are heat-treated in air for 3 hours at 350 ⁰ C in an electrically heated batch furnace. After the heat treatment, the weight of the samples is determined. The samples sawn from valves MV1 and MV2 are left in untreated state as reference material. The weight of these samples is also determined

A heat-treated sample from engine valve MV3 is examined by microscope. Figure 4 is an enlarged image of the sample. It shows part of the engine valve (on the right in the diagram) on which an iron chrome oxide layer has formed (the narrow white region to the left in the diagram). The thickness of the layer formed is measured as 10 μm and, as may be seen in Figure 4, the layer is tight, i. e. without pores. The hardness of the layer is measured in a Vickers hardness tester as 1500 HVO.1.

Thereafter the corrosion resistance of the heat-treated samples from engine valves MV3 and MV4 and the corrosion resistance of the untreated samples from the reference engine valves MV1 and MV2 are investigated.

The investigation procedure is as follows:

A solution of 720 ml of fully deionised water, 20 ml of sulphuric acid with density 1.84 g/cm3 and 25 g of iron (III) sulphate is prepared. The solution is brought to the boil at about 100 ⁰ C in four separate glass flasks provided with water-cooled cooler.

A sample from each engine valve MV1 , MV2, MV3 and MV4 is thereafter placed in the respective flask. The samples are heated in the solution for 60 minutes at boiling point, with return of vaporised water to the solution via the water-cooled cooler so that the concentration of the solution remains constant.

The samples are then taken out of the flasks and the weight of the samples is determined again. The weight loss of the respective samples is determined from the difference between their weight before and after treatment in the solution. The weight loss is the percentage weight lost by sample as a result of corrosion in the acid solution. The weight loss provides a measure of the corrosion resistance in that large weight loss means low corrosion resistance and small weight loss means high corrosion resistance. Table 1 shows the weight loss of the samples after the corrosion experiment.

Table 1 : Results of corrosion experiment

Table 1 shows a weight loss of only 0.01 to 0.02 percent by the samples from the heat-treated engine valves MV3 and MV4. In contrast, the samples from the untreated engine valves MV1 and MV2 have lost about fifty percent of their weight as a result of corrosion in the acid solution. The results of the corrosion experiment therefore show that the iron chrome oxide layer formed at low temperature protects engine valves MV3 and MV4 against corrosion in an acid environment.

The invention described above may be given various alternative embodiments within the scope of the claims set out below.