US2170268A - Alloy steel for the manufacture of motor valves - Google Patents

Description

Patented Aug. 22, 1939 UNITED STATES PATENT OFFICE ALLOY STEEL FOR THE MANUFACTURE OF MOTOR VALVES No Drawing.

Application March 1'7, 1939,

Serial No. 262,447 1 Claim. (Cl. 75-128) This invention relates to alloy steel for the manufacture of valves for use in internal combustion engines and commonly called motor valves, and its principal object is to provide an 5 alloy steel at a relatively low cost for the manufacture of motor valves, and particularly the stems of plural piece motor valves, and capable of continuous eflicient use in high speed motors made in accordance with the present day practice. I 9

Motor valves constructed of alloy steel, forming the subject matter of this specification, have greater tensile strength, wearing qualities and high fatigue resistance, are not susceptible to II chemical or acid reaction from the gases developed in motors and are non-scaling, tougher and stiffer at elevated temperatures than valves heretofore used in high speed motors.

The present alloy steel is intended principally for the manufacture of two piece and three piece valves, of which the stems thereof are composed of an alloy steel containing the hereinafter specified analysis.

A complete description of the construction of u a three piece motor valve will be found in my prior Patent No. 2,037,340 dated April 14, 1936, for improvements in a Composite article of manufacture, to which reference may be made.

In motor valves requiring a higher. grade of go material for the exterior of the head thamis required for the stem, the latter may be formed from the alloy steel forming the subject matter of the present specification. In two'piece forged motor valves, a rod composed of one part formed 35 of the present alloy metal and another part composed of the higher grade of material are buttwelded together and the head and neck thereafter forged into a valve in the customary manner. 1

40- Trm GENERAL REQUIREMENTS or STEELS roa Moron Exrmps'r VALVES AND VALVE STEMS Due to the increased power of modern m'otors, doped fuels, and higher speeds, valve steels have been called upon to withstand a combination of heat, corrosion and stresses far in excess of any other parts of the engines. Only a limited number of steels are able to withstand motor operating conditions satisfactorily.

50 Valve steels should meet certain general requirements. While each requirement is desirable, no steel has yet been found to satisfy all of them. Below are the more important properties which are considered pertinent in the selection of good valve steel: i

(1) Resistance of high temperature oxidation and corrosion (2) Permanence of properties after repeated heating and cooling Another important phase which must be considered in valve steels is the effect of alternate heating and cooling, on the material. A valve steel must be capable of being heated to the maximum temperature that it may attain in any particular motor and yet retain its physical properties at that temperature for a long period of time. In order to meet this requirement, the 5 critical temperature should be as high as possible. If the steel passes through its critical range each time it is heated and cooled, there will be a decided change in size and also in physical characteristics which would probably result in warping and eventual failure. V

A valve steel must also have a stable structure at high operating temperatures, that is, its physical properties must not be changed appreciably by continuous heating at motor operating temperatures. When some steels are heated for long periods of time at temperatures below the critical, spheroidization takes place with a resultant loss in strength and decrease in resistance of oxidation and corrosion. The above efiect can 40 only be minimized by providing as great a difference as possible between the critical temperature of the steel and the valve temperature and through the addition of stabilizing elements.

With regards these factors, experimental tests have proven that valve stems made of the present alloy steel are very satisfactory. The stems in tested valves show no indication of oxidation or corrosion and they show less carbon build up on the stem just above the stem guide level than usual. This is a decided asset because the possibility of carbon falling into the clearance between the valve stem and valve stem guide is minimized and, therefore, subsequent scoring of the stem is decreased.

arc welding process.

(3) Heat conductivity and thermal expansion A large portion of the heat stored in a valve head is dissipated by conduction down the stem where the heat is transferred to the valve guide. The life of a valve is, therefore, dependent on the heat conductivity of the valve stem material.

Closely associated with heat conductivity of a valve stem is its thermal expansion. Although this factor is not actually considered in the selection of. a valve steel, it has much bearing on the determination of the clearance required between the valve stem and valve stem guide. The rate of heat transfer from the stem to the guide is increased as the clearance is decreased. However, the minimum clearance must insure a sliding fit there must be no binding. A large clearance diminishes the transfer of heat until the valve has reached its operating temperature.

(4) Strength and toughness at elevated temperatures Steels possessing high strength and toughness at the elevated temperatures of operating motors are unquestionably the most desirable for use as valves and valve stems. It is also necessary that a valve steel have a high yield point. The present alloy steel has both a higher tensile strength and a higher yield stress at the elevated temperatures attained in modern motors.

In order to serve satisfactorily as a valve or valve stem material, a steel must be tough at motor operating temperatures.

A property closely related to the toughness of a valve steel is its fatigue strength at elevated temperatures. Simple fatigue is not of serious concern in the prevention of valve failure. However, fatigue stress coupled with the action of the hot, corrosive and oxidizing exhaust gases often leads to premature valve failures which have been attributed to a high temperature corrosion fatigue" phenomenon.

(7) Weld ng characteristics Only easily welded steels can be employed as valve stems in three piece exhaust valves. The stem in this type of valve must be welded to the steel cup and head metal by means of a carbon In. two piece valves, the upper and lower portions of the stem are made of two difierent steels butt-welded just below the level of the valve stem guide. From practical experience, I have determined that the present alloy steel can be much more easily welded than many other valve steels now on the market.

(8) Heafireatm characteristics In order to obtain the best combination of surface stability, strength and toughness, valve steels must be heat treated. In addition, three piece welded exhaust valves require heat treatment to refine the coarse grain in the top of the stem which is the result of the high temperature developed in the process of carbon arc welding the stem to the cup and heat metal. After welding, the coarse grain size in the stems .can be effectively refined by a simple normalizing process. The strength, toughness and surface stability are greatly enhanced by reheating to a temperature just below the critical and quenching into wate In the case of valve stems made of two pieces of dissimilar steels butt-welded together, heat treatment is necessary to develop the best physical and chemical properties at the welded point. The present alloy steel can be butt-welded very ing the temperature of the valve. very large guide clearance may cause the valve readily to a great variety of low alloy steels and subsequently heat treated to give a bond of. high strength and toughness.

The ends of valve stems must withstand the continuous pounding of tappets. This can only be accomplished by hardening the end of the stem. The present alloy steel lends itself nicely to burn hardening and a suitable hardness is developed at this point to withstand the blows of the tappet mechanism.

(9) Wearing qualities For most eflicient operation, the wear of valve stems and valve seats must be a minimum. Excessive wear'of the stem should be avoided for several reasons. Firstly, the rate of heat transfer to the stem guide is decreased, thereby rais- Secondly, a

to close on an angle, allowing one side of the head to hit the block first. This side seating can easily snap off the head from the stem. Too

' large a guide clearance also results in oil losses and inefiicient motor operation.

Motor tests indicate that the present alloy steel has excellent wearing qualities. In use, a valve stem made of the present alloy steel develops an extremely smooth surface of high luster. In this respect it is superior to many other valve steels.

(10) Forgealnlity Carbon 0. to 0.30 Chromium 5. 25 to 7. 00 Silicon 3. 00 to 4. 00 Nickel 1. 5 to 2. 10 Manganese 0. '70 to 1. Sulphur Less than 0.03 Phosphorus Less than 0.03 Iron"; Remainder The specific analysis which I aim to maintain in obtaining the above mentioned results in the most efllcient maner is, as follows:

- Per cent Carbon"; 0.

Chromium 6. 25

Silicon. 3. 25

Nickel'n- 1.90

Manganese 1. 00'

. Sulphur 0.025 Phosphorus 0. 025

Iron Remainder In order to obtain the best combination of physical properties and to render the alloy steel resistant to the oxidizing and corrosive hot exhaust gases, the three piece motor valves must be heat treated. The best results have been obtained with the following heat treatment:

1. After the three piece motor valves are fabricated, (usually with a carbon arc welding process) the valves are cooled slowly in a furnace maintained at about1300 degrees F. until they have reached the furnace temperature. This step requires about fifteen minutes.

2. The valves are then cooled in still air.

-3. The valves are then reheated to about 1800 degrees F., held at this temperature for approximately ten minutes and then cooled in air.

4. The valves are then reheated to about 1525 degrees F. (this temperature being below the critical of the steel), then held for about ten minutes and quenched in water. This heat treatment imparts to the valve stems great strength and toughness, both at room temperature and at temperatures reached by a motor valve during operation in a motor.

Two piece motor valves, of which the stem is composed of the present alloy steel, require no special heat treatment although at the buttwelded ends they are normalized to relieve the strains that may have been set up due to the butt-welding process.

It is a common practice to test motors at the present time at a speed of from 4,600 R. P. M. to above 5,000 R. P. M. and in many cases requiring a spring pressure of up to 140 pounds to seat the valves. The well known Silchrome steel valves when elevated to a temperature of 1,400 degrees F. show a maximum tensile strength of only 11,000 pounds which is not suflicient to meet the requirements in the present day high speed motors. They do not have the high fatigue resistance required and are likely to fracture under the severe strain to which they are put in the pressent day high speed motors.

Tests made with motor valves fabricated from my new alloy steel show that they have a tensile strength of 18,000 to 20,000 pounds at 1,400 degrees F. against 11,000 pounds in valves made of Silchrome or other high grade alloy steel and, further, that the non-scaling property is as high as that of Silchrome when heated to 1,400 degrees F. The result is that when the present motor valves are used in the present day high speed motors having a speed of from 4,600 R. P. M. to 5,000 B. P. M. and used in conjunction with powerful springs to seat them, they stand up in practice, are long lived, have high fatigue resistance, (10 not require frequent regrinding, seat perfectly and do not warp or wear rapidly. The endurance of the valves when subjected to the violent pounding of the valves on their seats, even when subjected to the high temperatures developed in motors, is unlimited. The fatigue resistance is far above that of any of the higher priced alloy steel valve when used in present day high speed motors.

It is to be understood that the present alloy steel is intended only for the manufacture of the stems of motor valves, which valves are subjected to very high temperatures during their service. Many articles composed of an alloy containing chromium, silicon or nickel contain some of the properties attributed to the present alloy steel, but they are not usable in the manufacture of motor valves that are subjected .to high temperatures.

I claim as new and desire to secure by Letters Patent:

An alloy steel for the manufacture of articles capable of continued eflicient use at high temperature composed of carbon 0.25 per cent, chromium 6.25 per cent, silicon 3.25 per cent, nickel 1.90 per cent, manganese 1.00'per cent, sulphur 0.025 per cent, phosphorus 0.025 per cent and iron the remainder.

GEORGE R. RICH.