EP0296439B1 - Austenitic steel for valves of internal combustion engines - Google Patents

Description

The invention relates to an austenitic, high-strength and hot corrosion-resistant steel for gas exchange valves of internal combustion engines.

The use of austenitic steels for the production of gas exchange valves with chromium, nickel and manganese as well as contents of tungsten, molybdenum and vanadium or with parts of niobium / tantalum, titanium, aluminum and cerium as well as corresponding contents of carbon and nitrogen is known per se and is known by numerous property rights are documented. For example, the following previous publications are mentioned:
DE-PS 9 34 836
DE-AS 10 44 131
AT-PS 2 66 900
U.S. Patent No. 24 96 245
U.S. Patent No. 24 95 731
U.S. Patent No. 26 03 738
U.S. Patent No. 2,657,130
U.S. Patent No. 26 71 726
U.S. Patent No. 28,39,391
DE-PS 25 35 516
U.S. Reissue PS 24,431

The powder metallurgical production of austenitic steels is also known. For example, reference is made to the publication "Powder metallurgically manufactured products in stainless steel" from Avesta Nyby, Torshälla, Sweden.

It is also known that powder metallurgy succeeds in producing very high-alloy steels which, because of their alloy structure, are difficult to produce in a conventional way via ingot casting, forging and rolling and with little output or no longer at all.

Powder metallurgy offers the possibility of cold compacting these high-alloy materials as atomized powder and then extruding them into bars with almost finished dimensions during hot extrusion.

Common to the precipitation-hardenable, highly heat-resistant, hot-corrosion-resistant valve steels with contents of carbon, nitrogen, chromium, nickel and manganese as well as possibly of tungsten, molybdenum and vanadium or also additionally niobium / tantalum, titanium, aluminum and cerium that their improved mechanical properties can only be achieved through a Heat treatment for precipitation hardening can be achieved, an optimization of the mechanical strength values generally being achieved if solution annealing is carried out before the precipitation hardening.

However, the optimized properties set by the usually very expensive heat treatments move outside the thermodynamic solution equilibria of conventional steels, so that their mechanical properties deteriorate again as the thermodynamic equilibrium is approached. The properties generated by heat treatment therefore only have a temporary lifespan if the temperatures and times progress during engine operation of the excretion reactions, which generally applies to the high stresses on the valves in modern, low-consumption internal combustion engines.

The corrosion-chemical properties of the aforementioned precipitation-hardenable valve steels are also optimized with solution annealing, because only then does the high chromium content in the matrix required for corrosion protection go into solid solution. During precipitation hardening of valves annealed during production, either immediately during manufacture or later during engine operation, the hot corrosion properties deteriorate because chromium is then preferably fixed in the precipitates and the finally coagulating precipitates and is removed from the alloy as a corrosion-protecting element.

Depending on the level of the thermal load in the engine, the steels heat-treated with great effort through solution annealing and precipitation hardening approach more or less quickly and completely and generally uncontrolled the thermodynamic solution equilibrium, which corresponds to the aging condition of the respective steel. In this state, the strength-increasing precipitates formed as precipitates by solution annealing and precipitation hardening are so largely aged that the coagulated precipitates can no longer contribute to increasing the strength and also impair the resistance of these steels to hot corrosion.

Starting from this prior art, the object of the invention is to create a steel alloy of the type mentioned in the introduction, the properties of which can hardly be changed by the action of heat, in particular by the thermal stresses of the engine.
The solution to the invention problem is characterized by a Steel manufactured by powder metallurgy with the following alloy additives in mass percentages: carbon 0.30 - 0.70 Silicon 0.50 max. manganese 8.00 - 16.00 chrome 24.00 - 32.00 nickel 8.00 - 16.00 molybdenum 2.00 - 5.00 Niobium / tantalum 1.50 - 4.00 Vanadium 0.30 max. nitrogen 0.30 - 0.70 Balance iron, as well as manufacturing-related usual impurities

Although it had to be expected that a steel within the limits of this composition range would be difficult to produce conventionally by block casting, forging and rolling, it was generally not to be assumed that the valve steel proposed according to the invention would have very special properties and advantages. It was found that his behavior can be changed very little by the effects of heat; because neither by solution annealing and precipitation hardening, nor by thermal stresses in the engine could its properties be influenced more than insignificantly. This means a considerable practical advantage. Gas exchange valves made from this steel can be used without any concerns without heat treatments such as solution annealing and / or precipitation hardening.

Further measures which advantageously supplement the invention are contained in the subclaims.

The steel with the proposed composition and produced by powder metallurgy has virtually unchangeable stable mechanical properties in all in the heat treatment Motor thermal stresses to be expected. In addition, its superior corrosion resistance is not only created by the heat treatment solution annealing, but is already predetermined without this and other heat treatments due to the advantageous alloy structure and the homogeneous distribution of the alloy elements due to the extremely fine-grained structure by the powder metallurgical production. The good corrosion resistance is therefore not affected by the thermal stresses in the engine, which was previously not considered achievable. Disadvantages of an economic nature, which cause the required heat treatments for the conventional steels, are avoided in the steel according to the invention.

In addition, the steel produced according to the invention has heat resistance and wear properties without heat treatment, which are partly due to the high carbon and nitrogen content, but also to the alloy-related matrix strengthening mentioned, and apparently due to the fine grain and the extraordinarily fine carbide and nitride - Distribution are supported. The high carbide and carbonitride content in the fine structure, which is in the finest distribution, leads, despite the lack of strength-enhancing heat treatment, to such high wear resistance on the abrasively loaded valve seat that this steel appears to be particularly suitable for unarmored gas exchange valves for diesel engines on the valve seat, for one - and exhaust valves to be used.

It is particularly important that the corrosion-chemical properties could be optimized at the same time by the proposed alloy structure in connection with the powder metallurgical production despite the lack of solution annealing. With very good mechanical properties, it was thus possible to use steel with very low corrosion rates, both in the case of an oxidative and an attack in a sulfur-containing atmosphere to realize. In the aforementioned known steels with a similar alloy structure, either high strength properties could be achieved by solution annealing and precipitation hardening with only moderate hot corrosion properties, or high hot corrosion resistance was only possible at the expense of mechanical strength. This means that both properties have hitherto largely been ruled out for the previously known steels of the alloy group mentioned.

In the tables below, the strength properties and corrosion rates of the steel according to the invention in the case of oxidative attack are compared with the corresponding values of previously known steels with a similar alloy structure. The selected embodiment of the steel according to the invention had the following chemical composition in mass percent:
C 0.53
Si 0.40
Mn 9.9
Cr 24.6
Ni 9.6
Mon 3.03
V 0.05
Nb 1.97
N 0.50 Weight loss in g / dm² due to corrosion attack in air treatment Alloy according to the invention Known alloy, e.g. 1.4785 750 ° C / 500 h 0.12 0.41 850 ° C / 500 h 0.60 2.83 950 ° C / 500 h 14.50 total loss of samples due to oxidation Weight loss in g / dm² due to sulfur corrosion treatment Alloy according to the invention Known alloy, e.g. 1.4785 870 ° C / 80 h 2.69 6.80 Properties at RT treatment Alloy according to the invention Known alloy z. B. 1.4785 Notched impact strength [J / cm²] Solution annealing: 1180 ° C, 30 ′ 50 40 LG + 2 h 760 ° C ≧ 38 3.5 LG + 2 h 800 ° C 28 <3 LG + 100 h 800 ° C 19th <2 without solution annealing 48 <2 * 2 h 760 ° C 42 <2 * 2 h 800 ° C 39 <2 * 100 h 800 ° C 16 <2 * Hardness HB 2.5 / 187.5 Solution annealing: 1180 ° C, 30 ′ 347 310 LG + 4 h 760 ° C 342 400 LG + 4 h 800 ° C 340 355 without solution annealing 350 > 400 * 4 h 760 ° C 342 > 400 * 4 h 800 ° C 335 > 400 * Tensile strength Rm [N / mm²] Solution annealing: 1180 ° C, 30 ′ 1100 1000 LG + 4 h 760 ° C 1100 1350 LG + 20 h 760 ° C 1100 1050 without solution annealing 1150 > 1350 * 4 h 760 ° C 1200 > 1350 * 20 h 760 ° C 1200 > 1100 * The well-known material z. B. 1.4785 cannot be processed in the non-solution-annealed state.

Claims (4)

1. An austenitic high strength hot corrosion resistant steel for the valves of internal combustion engines, characterized in that it is produced by powder metallurgy with the following chemical composition in percentages by weight: Carbon 0.30 to 0.70 Silicon 0.50 max Manganese 8.00 to 16.00 Chromium 24.00 to 32.00 Nickel 8.00 to 16.00 Molybdenum 2.00 to 5.00 Niobium/Tantalum 1.50 to 4.00 Vanadium 0.30 max Nitrogen 0.30 to 0.70 residue iron and usual impurities due to manufacture
2. An austenitic steel according to Claim 1, characterized in that the total of its carbon and nitrogen contents amount to at least 1.10% by weight.
3. An austenitic steel according to Claims 1 and/or 2, characterized in that its minimum chromium content is such that it can be determined from the carbon and nitrogen content by the following equation:
   
   $\text{\% Cr ≧ 15 + \% C x 15 + \% N x 3.5}$

   

   .
4. An austenitic steel according to one of Claims 1 to 3, characterized in that it additionally contains separately or together up to 2.5% by mass of alloying elements cerium, aluminium and titanium at the expense of the residual elements.