Precipitation-hardenable stainless steel method and product

[57] ABSTRACT

Method for developing great strength, along with ductility, in the martensitic precipitation-hardenable stainless steels, particularly bar, rod and wire products, such as the 17-4 PH grade (about 17 percent chromium, 4 percent nickel, 3 percent copper, 0.07 percent max. carbon, and remainder iron), wherein the product, moving at significant speed, is subjected to high electrical energy heating to quickly take the hardening ingredients (copper, carbon, etc.) into solution; then quenched to transform the metal and preclude excessive grain growth, and finally reheated to effect hardening and strengthening.

Claims, No Drawings • 3,660,176

PRECIPITATION-HARDENABLE STAINLESS STEEL METHOD AND PRODUCT

As a matter of introduction, my invention relates to the martensitic precipitation-hardenable stainless steels, especially in the form of bar, rod and wire stock, or even sheet and strip.

Among the objects of the invention is the provision of a method for producing stainless steel, particularly bar, rod, wire, sheet, strip and like products, of great hardness and strength, yet with good ductility, more especially the provision of a method for producing precipitation-hardenable stainless steel mill products of fine, equi-axed grain structure, in simple, rapid and efficient manner, with a bare minimum of scaling or surface discoloration; which steel of fine, equi-axed grain structure readily lends itself to simple precipitation heat-treatment to effect desired hardening and strengthening and the realization of products enjoying a combination of high strength along with good ductility and toughness.

Other objects of my invention in part will be apparent and in part more particularly pointed to in the description which follows.

The invention, accordingly, may be considered to reside in the combination of particular chemical composition of the steel and the succession of operational steps performed thereon, all as more particularly described herein and pointed to in the claims at the end of this specification.

BACKGROUND OF THE INVENTION

In order to gain a better understanding of certain features of my invention, it may be well to note at this point that the precipitation-hardening stainless steels are now rather highly developed. Particular reference is made to some of the more common martensitic grades, such as 17-4 PH (about 17 percent chromium, 4 percent nickel, 3 percent copper, and remainder iron) and 15-5 PH (about 15 percent chromium, 5 percent nickel, 3.3 percent copper, and remainder iron). Reference also is made to PH 13-8 Mo (about 13 percent chromium, 8 percent nickel, 2.25 percent molybdenum, 1.2 percent aluminum, and remainder iron) and PH 12-9 Mn 40 * ’ Γ *nvi y UUVilUHI, l.X percent aluminum, and remainder iron) and PH 12-9 Mo (about 12 percent chromium, 9 percent nickel, 2 percent molybdenum, 1.8 percent copper, 1 percent aluminum, and remainder iron). Additionally, there is Stainless W (about 17 percent chromium, Ί percent nickel, 1 percent titanium, and remainder iron), Custom 455 (about 11.75 percent chromium, 9 percent nickel, 2.25 percent copper, 1.2 percent titanium, 0.3 percent Columbium, and remainder iron) and 16-6 PH (about 15.25 percent chromium, 6.75 percent nickel, 0.4 percent aluminum, 0.8 percent titanium, and remainder iron). These steels in precipitation-hardened condition variously enjoy high tensile strengths, notably up to some 200,000 psi for the 17-4 PH, the 15-5 PH and Stainless W, and up to about 220,000 psi for the PH 13-8 Mo grade.

The differences between the ultimate tensile strengths of the several steels, as well as the differences in other mechanical properties, may be attributed to the differences in the ingredient added for obtaining the precipitation-hardening effect, e.g., copper in the 17-4 PH and the 15-5 PH, aluminum and molybdenum in the PH 13-8 Mo, and titanium in Stainless W, this in addition to the differences in the chromium-nickel balances, and further than this, in the structural differences in the solution-treated conditions.

While, as suggested, all of the steels enjoy considerable hardness and strength in the precipitation-hardened condition, there is an ever-increasing demand for stronger steels with a higher ratio between strength and weight, steels in which workability and ductility are not sacrificed to the increase in strength, and in which the superior combination of properties is not gained at the expense of higher costs.

Among the objects of my invention is the provision of a precipitation-hardening steel, together with a novel method of heat-treating the same, so as to enjoy maximum strength, along with good ductility, yet with an ease of handling in treatment, and all at minimum cost.

SUMMARY OF THE INVENTION

Giving now attention more particularly to the practice of my invention, I provide a method for treating the known martensitic precipitation-hardening stainless steels more particularly identified above so as to gain a quick heating of the metal to solution-treating temperatures, followed by a quick quenching, by reason of which there is had a precipitationhardenable steel of extremely fine grain size, at least ASTM 13 I and usually not exceeding ASTM 14.

In general, the steels contain about 10 to 21 percent chromium, about 2 to 11 percent nickel, with one or more of the ingredients copper about 1 to 5 percent, aluminum about 0.3 to 2 percent and titanium about 0.5 to 2 percent, with remainder substantially all iron. Where desired, there may be present molybdenum up to about 5 percent, cobalt up to about 10 percent and columbium up to about 1 percent. Carbon is present in amounts up to about 0.10 percent and manganese up to about 2 percent. A best combination of results is had in the steel containing about 12 to 18 percent chromium, about 4 to 9 percent nickel, with one or more of copper about 2 to 5 percent, aluminum about 0.4 to 1.5 percent and titanium about 0.7 to 1.5 percent. Here again, molybdenum may be added, this up to about 3 percent, and columbium up to about 0.5 percent. Carbon and manganese preferably are low, the carbon being up to about 0.08 percent and the manganese up to about 1 percent.

In the method of my invention the quick heating is had in such manner as to assure a virtual uniformity of heating and uniformity in the temperatures had, all with the result that a very fine grain structure is realized throughout the metal, certainly throughout the regions near the surface. Upon subjecting the solution-treated and quenched steel to aging or precipitation-hardening temperatures, there is had a steel of exceptional strength, along with good retained ductility and toughness.

More particularly, I provide a martensitic precipitationhardening stainless steel of the general character indicated above, this in the form of cold-drawn wire or bar or coldrolled sheet and strip. The wire size ranges up to some onehalf inch in diameter, with bar up to 1½ inches and the sheet and strip up to three-sixteenths inch thickness.

The cold-drawn or cold-rolled precipitation-hardenable I stainless steel product is paid off from a spool or reel upon which it has been wound subsequent to drawing. The wire, rod, sheet, strip or the like is led axially through an established zone of controlled high heat energy. More especially , I employ a zone of electric induction heating, or electrical resistance

I heating, or a combination of the two. The product then is fed into a quench tank and onto a wind-up reel. For the larger products a quench in water is desirable, but for the smaller or finer products a simple quench in air is sufficient to effect the desired quench.

Illustratively, a suitable form of apparatus is that described in the Rudd, et al. U.S. Pat. No. 2,932,502, of Apr. 12, 1960, entitled “Apparatus for Continuous Heat Treating of Wire”. A source of high-frequency electrical energy is applied directly to the steel product at two spaced points along its length, this effecting a direct resistance heating. In addition, the high frequency energy is applied to one or more electrically conducting tubes co-axially surrounding the product but insulated therefrom by suitable insulating liners. The conductive tubes serve to provide an electrical capacity effect between product and tube which assures an even distribution of high-frequency current. The source of energy employed is adjusted to some 10 kilocycles per second on up to about 450 kilocycles per second, this depending upon the size, that is, the width or thickness of the product being treated and upon the depth of current penetration desired. For the drawn or rolled products of finer gauge, that is, wire on the order of some 0.110 inch diameter or sheet and strip of that thickness, I adjust the source to about 450 kilocycles per second, this giving a current penetrating some 0.008 inch beneath the surface of the metal. For the coarser products the lower frequency is

3,660,176 desired, for example, I set the source at 10 kilocycles per second, this giving a significantly deeper penetration of current and in general amounting to some 0.053 inch. And in many instances the standard 60 cycles may be used, or even a direct current, as noted above.

The product is passed through the high-frequency heating zone at such speed as it relates to the length of heating zone as to give a time of heating which amounts to some 1 second to 15 seconds, or better yet, some 1 to 5 seconds, or in some cases 2 to 10 seconds. For a heating zone of 2½ feet in length the speed of travel required amounts to some 10 to 150 feet per minute for a heating of 15 seconds at the one speed down to 1 second at the other. With this treatment, the product is brought to a temperature of about 1,500° to 2,100° F. The rate of heating is seen to be about 100° F. per second for the slower speed of travel and longer heating period up to about 2,100° F. per second for the faster speed and shorter period of heating. Following the heating, the wire, rod, sheet or strip is quenched, as by spraying with water, and wound onto a suitable take-up reel.

Microscopic examination of the product reveals a grain size which is exceedingly fine and equi-axed. The size of grain is found to be ASTM 14. A less fine grain structure, of course, may be had by delaying the quench, that is, prolonging the 25 time at which the metal remains at solution-treating temperatures, thereby permitting significant grain growth. For a best combination of properties in the final age-hardened condition, this in terms of strength with ductility, it is the very fine grain size which is desired. 30

In my method the product is maintained at elevated temperature for such a short space of time that there is very little surface oxidation. Actually, I find that where the maximum temperature of 2,100° F. is reached the surface of the metal receives only a light, flaky, black scale. This is readily wiped off, although where desired a light pickling step may be employed using, for example, a nitric acid — hydrofluoric acid dip of, say, 20 percent commercial nitric acid and 3 percent commercial hydrofluoric acid, both in aqueous solution. At the minimum temperatures, that is, some 1,500° F., the sur- ⁴⁰face develops a light straw color, a discoloration which is readily removed by simple wiping. There appears to be no necessity for going to the expense of a controlled atmosphere, such as the use of cracked ammonia, as in the known bright- ₄₅annealing methods. And the results had are much more uniform than those achieved with the batch annealing processes commonly resorted to with the wire of the heavier gauges. A real and direct economy, therefore, is had in my process which is not enjoyed in those of the prior art. 50

Precipitation-hardenable stainless steel wire, rod, sheet, strip and the like, treated in accordance with my invention, commonly is supplied a customer-fabricator in the annealed or solution-treated condition. It is the customer-fabricator who ordinarily subjects the wire to the required aging treat- 55 ment.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

As particularly illustrative of the practice of my invention and the products had, wire of the 17-4 PH grade, cold-reduced ⁶⁰in the amount of 52.5 percent, this to a final size of 0.110 inch in diameter, was rapidly solution-treated by high-frequency induced current (450 KC resistance heating as shown in the Rudd patent referred to), water quenched and aged. The particular steel, identified as Heat No. 61261, analyzed carbon 0.033 percent, manganese 0.27 percent, phosphorus 0.019 percent, sulphur 0.015 percent, silicon 0.67 percent, chromium 16.22 percent, nickel 4.18 percent, copper 3.40 percent, with 0.23 percent columbium, and remainder iron. This is a steel which is martensitic in the solution-treated condition and is age-hardened by a mere heating at precipitation-hardening temperatures. Specimens of the wire were annealed or solution-treated at temperatures of 1,500° , 1,700°, 1,900° and 2,100°F. Some of the specimens were exposed to temperature for about 2 seconds, and others for about 10 seconds, these for a heating zone of some 3 or 4 feet in length, and at speeds of 98 feet per minute and 20 feet per minute, respectively, for the two groups.

The mechanical properties had for the 17-4 PH steel wire in the annealed condition, as well as those had for the wire in the aged condition, that is a reheating at 900° F. for 1 hour, are given below in Table 1(a), this for the specimens treated for about 10 seconds (run through the 4-foot heating zone at ¹⁰ about 20 feet per minute). Those for the specimens solutiontreated for a period of about 2 seconds (run through the heating zone at a rate of about 95 feet per minute) are given below in Table 1(b), both for the steel in the solution-treated condition and for the steel when subjected to the aging treatment of 900° F. for 1 hour. The properties reported are the ultimate tensile strength and the 0.2 percent yield strength, both in pounds per square inch (psi), percent elongation in 2 inches, percent reduction of area, and hardness in Rockwell C.

TABLE 1(a)

Mechanical properties ot 17-4 PH stainless steel wire, heat No 612G1 following 10-seeond solution-treatment and following subsequent aging treatment ¹

<td></td><td> 2%</td><td> Per-</td><td> Per-</td><td> Hard-</td>

<td> UTS,</td><td> YS,</td><td> cent</td><td> cent</td><td> noss,</td>

<td> p.s.i.</td><td> p.s.i.</td><td> elong.</td><td> R.A.</td><td> Re</td>

As received.._________________

As quenched from 1,500° F _. And aged 900° F..............

As quenched from 1,700° F_. _ And aged 900° F__________....

As quenched from 1,900° F _ _. And aged 900° F..............

As quenched from 2,100° F...

And aged 900° F______________

209,000 192,000 172,000 143,000 220,000 220,000 175,000 151,500 230,000 230,000

188,000 172,000 230,000 229,900 181,500 164,000 247,000 246,000

22.7 54.0 41

18.6 68.9 37

20.0 55.8 ..

18.6 69.6 39

12.5 49.0 ......

21.9 52.8 40

15. 0 58. 8

16.3 65.8 41

20.0 52.5 ......

TABLE 1(b)

Mechanical properties of 17-4 PH stainless steel wire, heat No 61261 following 2-second solution-treatment and following subsequent aeiii» treatment

<td></td><td> UTS, p.s.i.</td><td> .2% ys, p.s.i.</td><td> Percent elong.</td><td> Percent R.A.</td><td> Hardness, Re</td>

<td> As received.................</td><td> .. 209,000</td><td> 192,000</td><td> 90 7</td><td> 54.0</td><td> 41</td>

<td> As quenched from 1,500° F.</td><td> . 192,000</td><td> 162,000</td><td> 25.0</td><td> 58. 8</td><td> 37</td>

<td> And aged 900° F............</td><td> .. 213,000</td><td> 213,000</td><td> 25.0</td><td> 62.4</td><td></td>

<td> As quenched from 1,700° F,</td><td> . 196,000</td><td> 172,000</td><td> 15.0</td><td> 59.5</td><td></td>

<td> And aged 900° F____________</td><td> . 230,000</td><td> 230, 000</td><td> 20.0</td><td> 58. 0</td><td></td>

<td> As quenched, from 1,900° F_.</td><td> . 197,000</td><td> 166,000</td><td> 10.0</td><td> 55. 5</td><td> 39</td>

<td> And aged 900° F________ .</td><td> . 242,000</td><td> 240,000</td><td> 20.0</td><td> 51.0</td><td></td>

<td> As quenched from 2,100° F__</td><td> . 190,000</td><td> 146,000</td><td> 20.0</td><td> 59.0</td><td> 40</td>

<td> And aged 900° F__________</td><td> . 246,000</td><td> 245,000</td><td> 20.0</td><td> 52.8 .</td><td></td>

For comparison purposes, there are presented below in Table 1(c) the mechanical properties of some five samples of 17-4 PH stainless steel wire of a size ranging from 0.150 inch diameter to 0.250 inch diameter, conventionally batch-heated in the form of 300-pound coils at a temperature of 1,900° F. for one-half hour to reach uniformity, and then aged at 900° F. for 1 hour. The properties reported are ultimate tensile strength and 0.2 percent yield strength, both in pounds per square inch (psi), percent elongation in 2 inches, and percent reduction of area.

TABLE 1(c)

<td colspan="5"> Mechanical Properties of 17-4 PH Stainless Steel Wire Following Batch Solution-Treatment at 1900° F. and Aging at</td>

<td colspan="4"> 900° F.</td><td rowspan="2"> % R.A.</td>

<td> Example</td><td> UTS psi</td><td> .2% YS psi</td><td> % Elong.</td>

<td> 1</td><td> 196,000</td><td> 195,000</td><td> 10.0</td><td> 55</td>

<td> 2</td><td> 202,000</td><td> 198,000</td><td> 13.5</td><td> 53.5</td>

<td> 3</td><td> 204,000</td><td> 200,000</td><td> 13.0</td><td> 58</td>

<td> 4</td><td> 205,000</td><td> 198,000</td><td> 10.9</td><td> 47</td>

<td> 5</td><td> 194,000</td><td> 173,000</td><td> 11.7</td><td> 53.3</td>

3,660,176

Study of the test data presented above in Tables 1(a) and 1(b), as compared to that presented in Table 1(c), immediately reveals the superior tensile strength, and at the same time superior ductility, had with the rapid solution-treatment, followed by the precipitation-hardening treatment in accordance with the teachings of my invention. For example, the aged steel according to my invention, with solution-treatment at 1,900° F. and aging at 900° F., has an ultimate tensile strength of some 230,000 psi for the 10-second solution-treatment and 242,000 psi for the 2-second treatment, this as against some 194,000 psi to 205,000 psi for the steel conventionally solution-treated at 1,900° F. and then aged at 900° F. The yield strength figures are even more in favor of the steel treated according to my invention. With my practice there is had a yield strength of some 229,900 psi for the aged steel having the longer solution-treatment and 240,000 psi for the steel having the shorter treatment, this as compared to some 173,000 to 200,000 psi for the conventionally treated and aged steel. And with the increase in tensile properties enjoyed, there also is gained an increase in ductility, the steel treated according to my invention showing an elongation of 15.0 and 20.0 percent for the two differing times of solution-treatment, as against a best elongation of 10.0 to 13.5 percent for the conventionally treated steel.

Note, also, from the test data set out in Tables 1(a) and 1(b) that the strength figures, as well as those for elongation and reduction of area, are about the same where aging is had following solution-treatment at 2,100° F. both for the very short time of treatment and for the longer period of treatment. It will be seen that with the longer time of solution-treatment there is had an ultimate strength of 247,000 psi and a yield strength of 246,000 psi as against an ultimate tensile strength of 246,000 psi and a yield strength of 245,000 psi for the steel receiving the shorter solution-treatment. The elongation figures are precisely the same for both, namely 20.0 percent. And the reduction of area amounts to 52.8 percent for the hardened steel solution-treated for the shorter period and 52.5 percent for the steel solution-treated for the longer period.

Almost equally good results are had for the steel solutiontreated at 1,900° F. for the shorter period, the ultimate tensile strength amounting to 242,000 psi and the yield strength 240,000 psi, with an elongation of 20.0 percent and a reduction of area of 51.0 percent. Solution-treatment at the lower 45 temperature of 1,500° F. for the shorter time, however, results in a loss of strength in the aged condition, although enjoying a gain in ductility. With solution-treatment at a temperature of 1,700° to 2,100° F., however, best results are had with the shorter treatment, particularly good results being had where 50 the solution, treatment is effected at temperatures of 1,900° and 2,100° F. for the shorter period.

It readily will be seen that the 17-4 PH steel treated in accordance with my method develops strengths which are substantially greater than those realized by conventional heat- 55 treating methods. Particularly, as noted above, there is had an ultimate tensile strength of about 230,000 to 247,000 psi according to my method where, with induction heating, there are employed solution-treating temperatures of some 1,900° to 2,100° F. for some 2 to 10 seconds, this as against the 60 205,000 psi tensile strength had in the 17-4 PH steel solutiontreated at 1,900° F. and precipitation-hardened according to conventional methods as previously noted.

Moreover, the short times of solution-treatment and the freedom from any necessity for bright annealing, yet achieving 65 a high-quality surface, assure substantial savings in the cost of treatment.

In conclusion, it will be seen that I provide in my invention a method and product in which the various objects hereinbefore stated are effectively achieved. In my method, I develop great 70 strength and hardness in the martensitic precipitation-hardening stainless steels, this through a combination of high-ehergy electrical heating for a very short period of time to gain rapid heating, followed by quenching and subsequent aging. Yet the steels prior to aging enjoy good workability, with good ductili- 75 ty. Particularly good results are had in the precipitationhardening steels, such as the 17-4 PH, the 15-5 PH, the PH 138 Mo and the PH 12-9 Mo more especially described above, steels which are martensitic when quenched from solution⁵ treating temperatures. And the lower temperatures are especially satisfactory in the rapid heating of steels which have been subjected to cold-working, e.g,, cold-drawing or coldrolling prior to heating as in the specific example given above.

Since many embodiments may be made of the method and product of my invention, and many changes made in the embodiments hereinbefore set forth, it will be understood that the description given above is to be interpreted as illustrative and not by way of limitation.