US2888373A

US2888373A - Method for differentially age hardening austenitic steels and products produced thereby

Info

Publication number: US2888373A
Application number: US609142A
Authority: US
Inventors: James M Cherrie; John E Mcmillan
Original assignee: Thompson Ramo Wooldridge Inc
Current assignee: Northrop Grumman Space and Mission Systems Corp
Priority date: 1956-09-11
Filing date: 1956-09-11
Publication date: 1959-05-26
Anticipated expiration: 1976-05-26

Description

y 1959 J. M. CHERRIE ETAL 2,888,373

METHOD FOR DIFFERENTIALLY AGE HARDENING AUSTENITIC STEEL-S AND PRODUCTS PRODUCED THEREBY Filed Sept. 11, 1956 2 Sheets-Sheet 1 mm;""1"" m W m,-

[UVEJWZUFE L/AMES M CHE/Pm: (/OHA/ L M'C/M/LLA/V May 26, 1959 M. CHERRIE ETAL 2,888,373

METHOD FOR DIFFERENTIALLY AGE HARDENING AUSTENITIC STEELS AND PRODUCTS PRODUCED THEREBY Filed Sept. 11, 1956 2 Sheets-Sheet 2 M Q yMH/Z ijiiited States Patent METHOD FOR DIFFERENTIALLY AGE HARDEN'- ING AUSTENITIC STEELS AND BRODUCTS PRO- DUCED THEREBY James M. Cherrie, Euclid, and John E. McMillan, Willowick, Ohio, assignors to Thompson Rama Wooldridge, Inc., a corporation of Ohio Application September 11,1956, Serial No. 609,142

9 Claims. .(Cl.'148'21'.54)

The present invention is directed to improvements .in the manufacture of articles suchxas poppet valves and the like for use in internal combustion engines.

The selection of a suitable material or materials for an exhaust valve in an internal combustion enginernecessarily involves a compromise. The head .of the valve, containing the seating face, requiresa .goodresistance to creep at elevated temperatures, high hot strength and hardness as well as corrosionresistance since-that portion of the valve is normally subjected to the corrosive attack of products of combustion in the engine. Gnthe other hand, the valve'stem itself, particularly at .the .tip end, requires a high degree of wear resistancesothat it will not deform under load. The resistance to wear .also in cludes the resistance to spalling which is a tendency of the material at the tip end to flake offduring use.

In order to achieve these characteristics, it has .previously been suggested that the valves should be composed of two diiferent types of alloys, the head end of the valve being composed of an austenitic material'and the stem end being composed of a ferritic material, with the two portions of the valve being weldedtogether and suitably heat treated. An austenitic material is, of course, a solid solution in which gamma iron is the solvent, and in which the crystals have a facecentered cubic lattice structure. .A'ferritic material on the other hand is one characterized by asolid solution'in which alpha iron is the solvent and which evidencesla .body centered cubic structure.

This method of combiningtwo .ditferenttypes .of steel is not completelysatisfactory because of the added cost incurred in processing due to the necessity of welding, and the multiple heat treatments required.

Attempts to produce the required properties .in the valves by starting with a completely'austenitic .or completely ferritic valve andheat treating havenot always proven successful because the heat treatment procedures employed frequently produced adequate creep resistance, for example, only at the expense of resistance to spalling, and vice versa. The need still remains therefore for a poppet valve structure so arranged that the physical properties of the valve in selected portions of the valve are best suited to resist the particular typesof stresses. and operating conditions which those portions are subjected to during use. The satisfaction of that need is theprincipal object of the present invention.

Another object of the invention is to provide an austenitic valve body having a head and a neck portion characterized by excellent creep resistance and a stern portion having substantially enhanced wear resistance properties.

Still another object of the invention is to provide a ice one piece austenitic poppet valve having improved stem and tip wear properties, and improved abrasion and deformation resistance.

A further object of theinvention is to provide a simplitied, economical method for producing articles of the type described.

It has now been found that improved physical characteristics can be achieved in articles such as poppet valves,

which are subjected to diverse types ofstresses and corrosive atmospheres during use by starting with an austenitic alloy and selectively heat treating the alloy .by a combination of solution heat treatment and aging to produce a one piece, integral valve having composite properties.

Heretofore, it was generally thought that the hardness of the valve at selected portions would determine, to a large extent, its ability to resist the stresses which those portions undergo during operation. However, we have now found that hardness is not a particularly important consideration insofar as resistance to spalling .is concerned, but that the important considerations in determining'the proper physical structure for the valve are .the microstructure and the grain size at those portions. It has been discovered that for anti-spallingproperties, the grain size should be very small, on the order of no larger than six on the standard ASTM test and preferably in the range from eight to ten. For proper hot strength and creep resistance, however, it has been found that the grain size should be greater than .that in the spall resistant portion ofthe article, so that the grain size in the valve head and valve neckforexample, should be at,

least as high asv six, preferablyv in therange from fourxto six.

Another important consideration is the nature of the microstructure produced. .As is Wellknown, .austenitic steels cannot be hardened by heating and. quenching but that such heating has an efiect upon. theconfiguration of the grains and also uponthe character of the carbideformations which exist. For example, an austenitic material which has been solution treated at 2150.F. for one hour, followed by a water quench and then aged at .1400 F. .for 14 hours, exhibits an austenitic matrix with a banded lamellar configuration of carbides. It has been determined that the presence of such lamellar bodies .de-

tracts substantially .from the .ability of the material to withstand wear, and is at least partlyresponsible.fortbe tendency to spall.

We have now found that substantially improved resultsv can be obtained by heat treating the portions of the valve which require creep resistance, such as the head and neck portions of the valve by induction heating at temperatures in the range from about 2150 F. for aboutS to 60 seconds. This solution heat treatment is thenfollowed by water quenching and an aging of the entire valve at a temperature in the range from about 1300 F. to 1425 F. for a period of from 5 to 14 hours. Temperatures less than about 1300" F., and down to about 1000 F. can be employed, but the times involvedforsuitable aging render such lower temperatures not feasible on a commercial basis.

The article which results from the combined heat .treatments mentioned above consists of a creep resistant head and neck portionhaving a grain size larger than six, and preferably from four to six, and a stem having a grain size no larger than six and preferably from eight toten.

Many dilferent types of austenitic steel can be employed for the purposes of the present invention. The following are specific examples of four such alloys:

Example 1 Percent Carbon .30-.45 Chromium 17.50-20.50 Nickel 7.00-9.00 Silicon 2.50-3.25 Manganese .80-1.30 Sulfur .030 max. Phosphorus .030 max. Iron Balance Example 11 Carbon .15-.25 Chromium 2000-2200 Nickel 10.50-12.50 Manganese 1.00-1.50 Silicon .70-1.25 Sulfur .030 max. Phosphorus .030 max. Nitrogen .15-.2O Iron Balance Example III Carbon .39-.52 Chromium 12.50-15.75 Nickel 12.50-15.75 Tungsten 1.50-3.00 Silicon .201.00 Molybdenum .20-.60 Manganese .85 max. Sulfur .040 max. Phosphorus .040 max. Iron Balance Example IV Carbon 0.475-.575 Chromium 20.00-22.00 Manganese 8.00-10.00 Nickel 3.25-4.50 Silicon .25 max. Sulfur .04-.09 Phosphorus .030 max. Nitrogen .38-.50 Iron Balance A further description of the present invention will be made in conjunction with the attached sheet of drawings in which:

Figure 1 is a fragmentary view in elevation, partly schematic, illustrating a heat treating assembly for solution treating the head and neck portions of a poppet valve;

Figure 2 is a fragmentary plan view of the assembly shown in Figure 1; and

Figure 3 includes a view of the completed poppet valve, and illustrates, in greatly enlarged form, the crystal struc ture of various portions of the valve at various times in the process.

As shown in the drawings:

in Figure 1 reference numeral indicates generally a poppet valve which has been produced by first extruding the alloy into a blank, followed by shaping the blank into a poppet valve structure which includes a relatively massive head portion 11 and a stem portion 12. The head portion 11 includes a tapered valve seating face 13, and a sloping neck portion 14 which connects the head portion 11 with the stem portion 12. The stem portion 12 also includes a lock retainer groove 16 near the tip end.

The poppet valve 10 is supported by means of a mounting bracket 17 on a conveyor belt 18 which is arranged to carry the poppet valve through a solution heat treating zone. In travelling through the zone, the head portion 11 and the neck portion 141 of the valve, as well as a relatively small portion of the stem immediately adjoining the neck portion 14 are heated by means of a plurality of induction heating electrodes in the form of elongated copper bars 19 and 21 positioned at opposite sides of the valve 10 and a third electrode 22 positioned above the valve head 11. The three

electrodes

18, 21, and 22 are connected by means of suitable conductors 23 to a source of high frequency voltage such as a generator 24. Since it is desirable to have the induction heating effect develop well into the body of the heat treated portions, the frequency of the applied voltage should be on the low side as far as induction heating processes are concerned. We have effectively employed frequencies on the order of eight to twelve thousand cycles per second, with a frequency of nine thousand cycles being preferred.

The temperature of the treated areas, as determined by an optical pyrometer is on the order of 2150 F. to 2500 F. Since the heating by this method is quite localized, the temperature at the lower portion of the stem does not rise more than three hundred degrees or so. The duration of treatment extends from about 5 to about 60 seconds, with longer times being used with lower temperatures, and vice versa. As a general rule, a temperature of 2300" F. for a period of 40 seconds has been found to be most suitable.

The crystal structure which results after the solution treatment is illustrated by the first structure appearing in Figure 3 of the drawings. This structure includes relatively massive grains of austenite 31 and small discrete particles of precipitated carbide 32. The thin, irregular lines such as line 33 are probably due to the presence of sulfides.

After the solution heat treatment, the entire valve body is then subjected to an aging treatment at a temperature in the range from about 1300 F. to 1425 F. for a period of from 5 to 14 hours. A preferred temperature is 1400 F. and the preferred time is 10 hours. As a result of this heat treatment, the microstructure and the grain size of the material are considerably modified, as evidenced by the second view appearing in Figure 3. This structure includes crystals of austenite 34 of a grain size of at least six and mostly in the range from four to six. The dark colored areas appearing in the drawing are attributable to nitrogen concentration in those areas.

The aging treatment when applied to the portions of the valve, such as the stem which has not been previously solution treated by induction heating results in the structure shown in the third view appearing in Figure 3. This reproduction is at the same magnification as the others. It will be immediately apparent that the grain size of the crystals 36 is considerably smaller in this portion of the valve than in the neck and head portions. Experimentally, it has been found that most of the grains, have a grain size in the range from eight to ten by the ASTM standard. This portion of the valve is also substantially free from lamellar carbide structure, which contributes substantially to its spall resisting properties.

The showing in Figure 3 also indicates the approximate hardness values of various portions of the valve, as measured on the Rockwell C scale. It will also be noted from an inspection of Figure 3 that the portion of the valve which is both solution treated and aged extends somewhat beyond the limit of the portion of the valve normally engaged by the valve guide. A convenient rule of thumb for determining the extent of the solution treatment is to employ solution treating to a length which is roughly five times the diameter of the stern portion, measured from the head of the valve.

Despite the fact that the hardness values at different portions of the valve do not vary substantially, the resistance to stress at elevated temperatures, and wear rehaving the composition of Example IV was solution treated by induction heating at a temperature of 2300 F. for 40 seconds, followed by aging at 1400 F. for hours. Another sample of the same material was heat treated solely by aging under the same aging conditions. Stress-elongation data taken on these two samples at a temperature of 1350 F., and at a stress of 10,000 pounds per square inch showed that the material which had been solution treated and aged required 683 hours for a 1% elongation, while that which had been aged only required twelve to fourteen hours for the same elongation.

The process of the present invention provides a valve which has several very distinct advantages. The portions of the valve are provided with a type of metallurgical structure which is best adapted to resist those operating conditions which the selected portions have to meet in operation. Because the relatively high temperature solution treatment is confined to the relatively massive head of the valve, less distortion occurs during fabrication of the valve. Less mechanical straightening is required, and there is less residual stress in the finished part. All of these factors mean that there is a lower tendency to deformation in the valve during engine operation.

It will be evident that various modifications can be made to the described embodiments without departing from the scope of the present invention.

We claim as our invention:

1. A one piece austenitic precipitation hardenable chromium-nickel steel poppet valve having a spall resisting hard stem tip of a grain size no larger than six and a creep resistant, neck portion and head portion having a grain size larger than that in said stem tip.

2. A one piece austenitic precipitation hardenable chromium-nickel steel poppet valve having a spall resisting hard stern tip of a grain size in the range from eight to ten and a creep resistant neck portion having a grain size in the range from four to six.

3. A one piece austenitic precipitation hardenable chromium-nickel steel poppet valve having a creep resistant structure including the head, neck, and a portion of the stem immediately adjacent said neck, said creep resistant structure being characterized by a grain size from four to six, and the remainder of said stem being characterized by a grain size in the range from eight to ten.

4. A method of heat treating an austenitic precipitation hardenable steel article to produce a structure having a creep resistant portion and a spall resistant portion, which comprises solution heat treating said article in the portions to be made creep resistant by induction heating at a temperature in the range from about 2150 F. to 2500 F. for about 5 to 60 seconds, quenching said article, and thereafter aging the entire article at a temperature of from 1300" F. to 1425 F. for a period of from 5 to 14 hours.

5. The method of heat treating an austenitic precipitation hardenable steel article to provide a structure havmg a creep resistant portion and a spall resistant portion which comprises solution heat treating said article in the portions to be made creep resistant by induction heating at a temperature'in the range from about 2150 F. to 2500 F. for about 5 to seconds, quenching said article, and thereafter aging said article at a temperature and for a time to produce a grain size of at least six in the portion requiring creep resistance and a grain size no larger than six in the portion of said article requiring spall resistance.

6. The method of heat treating an austenitic precipitation hardenable steel poppet valve which comprises induction heating the head and neck portions of said valve at a temperature in the range from about 2150 F. to 2500 F. for about 5 to 60 seconds, quenching said valve, and thereafter aging the entire poppet valve until the tip of the stem of said valve has a grain size no larger than six.

7. The method of heat treating an austenitic precipitation hardenable steel poppet valve which comprises induction heating the head and neck portion of said valve at a temperature in the range from about 2150 F. to

2500 F. for about 5 to 60 seconds, quenching said valve, and thereafter aging the entire poppet valve until the tip of the stem of said valve has a grain size of from eight to ten and said head and neck portions have a grain size in the range from four to six.

8. The method of making a poppet valve which comprises extruding an austenitic precipitation hardenable steel into a blank, shaping said blank into a poppet valve having an enlarged head portion, a stem portion, and a neck portion between said head portion and said stem portion, induction heating the head and neck portions only of said valve at a temperature in the range from about 2150 F. to 2500 F. for about 5 to 60 seconds, quenching said valve, and thereafter aging the entire poppet valve at a temperature in the range from 1300 F. to 1425 F. for a period of from 5 to 14 hours.

9. A one-piece poppet valve composed of an austenitic, chromium containing, precipitation hardenable steel having a spall resisting hard stem tip of a grain size no larger than six and a creep resistant neck portion and head portion having a grain size larger than that in said stem tip.

References Cited in the file of this patent UNITED STATES PATENTS Dyrkacz 'et al Sept. 25, 1956 Kegerise Mar. 11, 1958

Claims

3. A ONE PIECE AUSTENTIC PRECIPITATION HARDENABLE CHROMIUM-NICKEL STEEL POPPET VALVE HAVING A CREEP RESISTANT STRUCTURE INCLUDING THE HEAD, NECK, AND A PORTION OF THE STEM IMMEDIATELY ADJACENT SAID NECK, SAID CREEP RESISTANT STRUCTURE BEING CHARACTERIZED BY A GRAIN SIZE FROM FOUR TO SIX, AND THE REMAINDER OF SAID STEM BEING CHARACTERIZED BY A GRAIN SIZE IN THE RANGE FROM EIGHT TO TEN.