DE1608199B1 - USE OF AN AUSTENITIC STEEL ALLOY AS A MATERIAL FOR EXHAUST VALVES IN COMBUSTION ENGINES - Google Patents

Description

In order to increase the porosity of the ingot, a minimum manganese content of around 8% is required. In the steel used according to the invention, the manganese serves as Austenitizing agent primarily used to maintain a high nitrogen content in the steel. So that in the steel to be used according to the invention the above-mentioned specific nitrogen content from about 0.15 to 0.25% can be present, depending on the particular one Composition - the presence of manganese in a minimum amount of about 6.5 to not above 7.5% also required.

As already stated, the steel used according to the invention has austenite stability achieved primarily through the addition of carbon and nitrogen. However, since continuously increasing amounts of these elements solidify the austenite structure rapidly and thereby the Reduce the cold deformability of the steel accordingly, the steel should preferably just the Contain carbon and nitrogen quantities necessary to maintain a completely austenitic Structure is sufficient.

To the properties of the steel used according to the invention with regard to those discussed above To show properties, the following alloy was melted and then, as indicated below.

Example TabeUe I

Nominal compositions and analytically determined composition of a cold-formable test steel for valves

Sample no. Cr Ni Geha
Mn It the alloy
Si ig (weight pr
C. ocent)
N P. Nb B 54 #N
■ aA 21.0
22.30 5.0
5.15 7.0
6.70 0.5
0.49 0.20
0.20 0.20
0.213 0.08
0.07

- £ f N gives the nominal composition and
- £ f- A the real composition.

The above steel was given individual samples for hardness, creep and impact resistance tested in various heat treatment and other processing states. The results are contained in Table II below.

Tables
Mechanical properties of the cold-formable test valve steel

Heat treatment

No.

to the solution
treated ¹ )

Rockwell hardness "C"

honed
surface

cold rolled

50 ° / o
reduction

Aging (at 732 ° C)

100 hours

1000 hours

Creep strain ² )

732 ° C

703 kg / cm ²

% Creep /

Test duration

760 ⁰ C
Brinell hardness 3)
BHN ⁴ )

21.1 ° C
Impact resistance ³ )
mkg

B 54

16.0

20.8

45.1

37.5 35.7

0.15 / 100 hours

135.4

1.1

*) Treated as a solution:

1 hour at 1121 ° C, quenching with water.

² ) Description of the thermomechanical creep test: Treatment for the solution as in 1, grinding to a diameter of 1.9 cm, cold rolling in two passes to a thickness of 1.77 cm, stress reduction by tempering for 2 hours at 732 ° C.

³ ) Hardness, description, of the thermomechanical impact test:
Same as 2 above, aged 98 hours at 732 ° C.

o) BHN, 1000 kg load, 10 mm diameter, chrome carbide notch.

From the above values it can be seen that this steel meets the minimum requirements described above is sufficient for an ideal steel for exhaust valves of internal combustion engines. The steel B 54 is all Requirements regarding the minimum hardness at room and elevated temperature, creep strain and impact resistance.

It has been shown that this steel, in the aged state, has hardening at room temperatures which in the range of 35 to 38Rc (Rockwell "C") and is therefore well above the specified minimum of 27 Rc. It can also be seen that this steel has the lowest creep elongation values of 0.15%, which is far below the specified Minimum of 1%. The steel B 54 also has the highest hardness values at 760 ° C, namely 135 BHN, which is satisfactorily far above the stated minimum of 90 BHN. The following table ΙΠ shows the tensile properties at room temperature of that used according to the invention as exemplified above Stahls treated for solution at about 1149 ° C and quenched with water are shown.

Table ΙΠ
Tensile properties of the test steel

warmth
treatment
No. O, 2 * / o-expansion
border
(kg / cm ² ) train
strength
(kg / cm ² ) strain
C / ·) A
lacing
Where) B 54 4460 8200 53.5 65.2

For the above tests, machined tensile specimens were ground to a diameter of 1.28 cm. A 5.08 cm long valve was used to measure elongation.

From table ΙΠ it can be seen that the steel shown in the JgL - 5 example in the quenched and tempered condition, ie treated for solution and quenched, has relatively low tensile strength and high ductility, the latter, which leads to good cold deformability properties, both as a result of elongation as well as by necking. From these "0.2% proof stress and tensile elongation" it can be seen that these steels are characterized by a high degree of plastic deformation at room temperature. As shown in tables, the steel can be deformed by cold rolling without difficulty. It has However, it has been found that the steels which have a reduction in area of at least about 60% with a low yield point are most suitable for cold heading and extrusion into valves for internal combustion engines, since they are type 305 stainless steel, which in the quenched and tempered condition has a yield point of 2590 kg / cm ² and a tensile strength of 5950 kg / cm ² with 55 ^fl / o elongation at 5.08 cm and 65 ° / o constriction, approximately equal. As can be seen from table ΙΠ, the steel B 54 comes very close to these values with regard to a low yield point and high area reduction values.

Although the steels used according to the invention can be cold-worked at room temperature, the pressures required for the deformation are considerably reduced by preheating to about 232 to 871 ° C. or generally to temperatures below the temperature at which the recrystallization would otherwise take place during the deformation preferred temperature range extends below about 693 ° C. ^{In contrast to:} conventional "hot deformation" of valve steels at a forging temperature of around 1038 to 1204 ° C, this procedure could be called "hot deformation".

The steels used according to the invention are preferably produced in that the components melted in an electric arc furnace, tapped in ladles and then in Ingot molds are cast. The bars are reheated to approximately 1038 to 1204 ° C and, if Intended use for valves is hot rolled and cooled to room temperature. To the Cold or hot extrusion the billets are usually at 1121 to 1177 ° C, preferably 1149 ° C For 1 hour, or until all the carbides are in solution, solution annealed and then with sufficient Speed, for example by quenching with water, so that the carbides are in solution are kept, cooled to room temperature, where the hardness is about 10 to 20 Rc. The ingot will then cut into suitable pieces and compressed cold or warm into valve shape, the obtained Hardness is around 40 to 50 Rc. The valves are then set to about 2 hours at about 704 to 760 ° C, preferably at 732 ° C, stress relieved and cooled to room temperature, the hardness obtained being at least 35 Rc.

The valves are used for operation in internal combustion engines with an operating temperature of the exhaust valves of about 732 ° C used. A second hardening or aging hardening takes place in the valves during operation in such a way that a room temperature hardness above 30 Rc is maintained.

Claims (2)

1 2 valves of motor vehicles and the like Claim: to be. The subject of the present invention is therefore Use of an austenitic steel alloy - the use of an austenitic steel alloy, tion that at and below their recrystallization 5 those at and below their recrystallization temperature can be extruded, ratur can be extruded, consisting of consisting of 0.15 to 0.25% carbon, 19 to 0.15 to 0.25% carbon, 19 to 23% chromium, 23% chromium, 4.5 to 5.5% nickel, 6.5 to 7.5% 4.5 to 5.5% nickel, 6.5 to 7.5% manganese, 0.15 to Manganese, 0.15 to 0.25% nitrogen, each 0.05 0.25% nitrogen, each 0.05 to 0.2% niobium and / up to 0.2% niobium and / or tantalum and up to 1% io or tantalum and up to 1% silicon, the remainder iron and Silicon, the remainder iron and smelting-related impurities as plant impurities as a material for exhaust for exhaust valves in combustion power valves in internal combustion engines. machinery. The steel used in the present invention has a 15 balanced composition, which in terms of the limits of the essentials given above Elements is extremely critical. Detailed investigations have shown that the cold In the case of exhaust valves of motor vehicles and deformability and the other required internal combustion engines, the minimum shafts as mentioned above are then retained Requirements in terms of space and will be if the composition is so balanced High temperature resistance particularly strict. It becomes apparent that the steel is complete at room temperature is a cold-deformable steel for such ver or is substantially austenitic. These purposes of use regarding his physical examinations have further shown that the critical The following typical minimum dependence of the properties of the steel on the Requirements: The high-temperature tear-resistant structure can only be achieved by carefully balancing the speed, measured in 100 hours by the load Ni, Mn, C and N values obtained in the austenite for 1% elongation at 732 ° C, should be at least possible. 422 kg / cm ² and, as elongation at break at 732 ° C, the chromium content is measured within the critical 100 hours, at least 703 kg / cm ² within limits of 19 to 23%, so that one is applied. As far as hardness is concerned, it should be at least 90 Brinell for measured scaling resistance and 760 ° C and room temperature hardening at operating temperatures up to about 27 Rockwell "C". This is guaranteed by 760 ° C. Is the steel aged 704 to 760 ° C when a sample is heated in molten lead oxide, then Pb ₃ O ₄ at 913 ° C develops in it certain ■ oxidation resistance- 35 through the precipitation of carbides and nitrides, measured as weight loss (expressed in and, if the steel contains considerable amounts of phosphorus, grams) per area (dm ² ) and time (hour), ie in excess of about 0.04%, also (g / dm ² / h), should not be more than 60. The by the secondary or aging Charpy V notch test determined by the impact strength on hardening, determined by phosphides. The sulfur content of the steel should not be room temperature should be at least about 0.70 mkg 40 above 0.1%, preferably not above 0.04%. W Furthermore, the cold deformability should increase. that of a 305-type stainless steel, silicon commonly used in these steels as (18 Cr-12Ni-0.12 C) should be comparable. Contamination can contain up to one The French patent specification 1284728 shows a content of 1%. an oxidation-resistant austenitic steel alloy 45 The nickel content should ensure the tion known, which from 12 to 25% chromium, kept 5 to the necessary cold formability as high 30% nickel, 0.3 to 20% manganese, 0.2 to 3% are economical as it is affordable Tantalum, up to 0.2% carbon, up to 0.2% nitrogen. The studies have shown that the up to 2% silicon, the remainder iron including ersehmel lower limit for the nickel content with regard to there is contamination caused by the application. . 5p the required cold formability at around 4.5% In this publication, however, is only on the and that on the other hand, however, the nickel content Breaking resistance of this alloy referred to, which goes beyond 5.5%, achievable increase men. An indication of the use of such would take the cold formability is not so great Alloy for the manufacture of exhaust valves. . the associated increase in the cost of the alloy of motor vehicles and other combustion 55 to justify. machines can be found in this publication. In the steel used according to the invention is for not. Achieving an austenite structure neither a high Furthermore, from the Canadian patent specification nickel, a high nickel-manganese content is found 582725 fully austenitic steels known which are required from. Rather, it becomes more effective and cheaper 0.08 to 1.50% carbon, 12 to 30% chromium, 60 due to the relatively high proportions of carbon and 2 to 35% nickel, 3 to 12% manganese, 0.06 to nitrogen. Differentiate in this regard 0.60% nitrogen, not more than 0.45% silicon, until the steels used according to the invention are fundamentally 0.20% sulfur, the remainder iron. In this legend of austenitic steels with lower Publication is already the usability of carbon content, with z. B. 0.05% maximum carbon of these steels for the manufacture of valves made of 65 material, which are manufactured exclusively by adding nitrogen combustion machines described. be strengthened and with whom to achieve a full In contrast, according to the invention, a consistent austenite structure should have a minimum content of improved material for the production of exhaust - about 14% nickel plus manganese as well as to prevent