US3399085A - Method of nitriding - Google Patents

Description

METHOD OF NITRIDING Filed Dec. 22, 1965 lst. Step Preferred Range lst. Step 200'. Step 55 kzmummm ACTIVITY or NITROGEN, PNHJ/PHZJ/Z (P in Atmospheres) 550 560 570 TEMPERATURE, 0.

United States PatentO 3,399,085 METHOD OF NITRIDING Herbert E. Knechtel, Monroeyille Borough, and Harry H. Podgurski, Greensburg, Pa., assignors to United States Steel Corporation, a corporation of Delaware Filed Dec.22, 1965, Ser. No. 515,548 6 Claims. (Cl. 148-16.6)

ABSTRACT OF THE DISCLOSURE Method of nitriding steel surfaces in whichsurface is treated with a binary mixture of ammonia 'and hydrogen at an elevated temperature and atmospheric pressure. The ammonia decomposes and its nitrogen combines with alloying elements in the steel ,to form nitrides. Conditions are controlled to prevent nucleation of iron nitride, thus avoiding formation of objectionable White layer, which otherwise must be machined off the nitrided surface.

This invention relates to an improved method of nitriding steel.

It is known that a hardened case can be formed on the surface of certain types of steel by 'a nitriding process. Steels suitable for nitriding contain several alloyed elements, commonly aluminum, chromium and molybdenum, and sometimes nickel or vanadium. The conventional procedure is to pass cracked ammonia over the surface of a nitriding steel at a temperature of about 520 to 560 C. Some of the ammonia decomposes on the surface of the steel, and nitrogen thus released diffuses into the solid phase and combines with some of these alloyed elements to form nitrides. After a sufficiently long treatment, the nitrides produce a hardened case to a depth of several thousandths of 'an inch. Any molecular nitrogen (N generated during nitriding or present initially in the cracked ammonia atmosphere has such low nitrogen activity (hereinafter defined) that it can be ignored as a nitriding agent. The presence of N need only be considered in calculating the effective nitrogen activity of the nitriding atmosphere insofar as it affects calculations of partial pressures of NH and H Previous nitriding processes were conducted under conditions which produce a brittle outer skin on the case, known as white layer or damage. In the white layer some iron nitride is formed, along with nitrides of the alloyed elements. Within the rest of the case, only the alloyed elements form nitrides. Because the white layer tends to spall, it is often removed, as by grinding or chemical treatment, before the nitrided part is placed in service. For many pumposes a nitrided case is prepared with a hardness of at least 1000 DPH and a depth of at least 20 mils. The belief has been that a case of this hardness and depth can be obtained only under conditions which form 'a white layer. For a more extended discussion of this problem, reference may be made to Floe Patent No. 2,437,249.

An object of our invention is to p rovide an improved method of nitriding steel surfaces in which we obtain a case of hardness and depth at least as great as in previous methods, but we avoid altogether formation of a white layer.

A more specific object is to provide an improved nitriding method which we may conduct in either one or two stepse and in which we circulate ammonia-hydrogen mixtures over the surface of a nitriding steel, but control both temperature and nitrogen activity throughout the process to avoid formation of a white layer, yet obtain a case with desired characteristics.

In the drawing:

FIGURE 1 is a graph which shows the relation be- "ice 2 tween nitrogen activity and composition of (A) binary gas mixtures of NH and H (B) dissociated NH, at different volume percentages of dissociation, and (C) dissociated NH at different volume percentages of N2+H2; and v v,

FIGURE 2 is a graph which specifies the nitrogen activity we maintain in the nitriding gas atmosphere for any temperature we use during the optional second step of our method.

We have observed that the most reliable guide-to the nitriding power of .a gas formed by dissociation of ammonia is its nitrogen activity asdetermined by the formula: Y Nitrogen activity:

Partial pressure of NH in atmospheres (Partial pressure'of H in atmospheres) For example, the nitrogen activity of a binary mixture composed of 50% by volume NH and 50% H at one atmosphere pressure is:

A mixture of NH H and N at one atmosphere formed when parts of NH are 50% dissociated according to the reaction:

consists of 50 parts by volume NH 75 parts H and 25 parts N parts in all. The partial pressures P are represented by the fractions:

The nitrogen activity of this gas mixture at one atmosphere pressure is:

The nitrogen activity of a gas which consists of 50% by volume NH and 50% by volume H +N as dissociation products of cracked ammonia at one atmosphere absolute pressure is:

In the last example P has the value shown for the reason that H constitutes 75% by volume of the product for each mole of NH which is cracked.

In FIGURE 1 curve A shows the nitrogen activity of different binary mixtures of NH and H at one atmosphere pressure. Curve B shows the nitrogen activity for dissociated NH at different percentages of dissociation, likewise at one atmosphere pressure. Curve C shows the nitrogen activity for different volume percentages of N +H in a dissociated NH mixture at one atmosphere pressure. (Volume percent of N +H is commonly but erroneously referred to as the degree of NH dissociation.) To produce a nitrogen activity of any chosen value, we use as a nitriding gas a binary mixture of NH, and H in a volume ratio which may be determined from curve A.

According to our method we nitride 'a steel surface under conditions which altogether avoid iron nitride nucleation on the surface of the steel. We begin our nitriding operation by passing over the steel surface, at substantially atmospheric pressure and at a selected temperature between about 475 and 530 C., a binary mixture of NH, and H which has a nitrogen activity of about 0.5 to 1.8. By reference to curve A of FIGURE 1 it is seen that a binary mixture which consists of about 30 to 55 percent by volume NH and the remainder H possesses a nitrogen activity within this range. For each specific combination of temperature and nitrogen activity and each. specific steel, we continue this treatment for a time 'a little shorter than that which leads to nucleation of a white layer. We determine the nucleation time experimentally. For example, with one commonly used nitriding steel (Nitralloy 135M), if we operateat the upper limits of temperature (530 C.) and nitrogen activity (1.8), we must limit the nitriding time to less than 17 hours to avoid white layer completely. At lower temperatures or lower nitrogen activities, we can prolong the treatment proportionally.

Our preferred temperature is about 500 C., and our preferred nitrogen activity about 0.6 to 1.8 (34 to 55 percent by volume NH in the binary gas mixture). At 500 C. and a nitrogen activity of 1.17, we must cease nitriding the aforementioned steel within about 40 hours. A 30-hour treatment under such conditions produced a case which has a surface hardness of about 1100 DPH and 'a depth of about 16 mils. At 500 C. and a nitrogen activity of 0.6, we find no limiting nitriding time up to 260 hours. A 65-hour treatment of the same steel under the last mentioned conditions produced a case which has a surface hardness of about 1100 DPH and a depth of about 18 mils. At 260 hours we attained a surface hardness of about 1100 DPH and a case depth of about 30 mils.

The single nitriding step just described produces a case-hardened steel which may be used as a finished product for many applications. If only a shallow case is needed, we can obtain a useful product under any set of conditions within our broader limits, as long as we stop before we nucleate a white layer. If a deeper case is needed, we can operate within our preferred limits and prolong the treatment. Thus our single-step treatment can produce a case of any desired depth only at the expense of a rather long nitriding time. To obtain a case of comparable depth in a shorter time, we can of course change the conditions. during this step, starting at a higher nitrogen activity, and lowering it before we nucleate a white layer, but staying within the broader limits. Preferably we may use a two-step method.

In the optional second step of our method, we further treat the partially nitrided surface with a binary mixture of NH;; and H at a higher temperature but at a lower nitrogen activity. In this step we treat the surface at substantially atmospheric pressure under conditions which approach, but lie below, the curve shown in FIG- URE 2. That is, we employ nitrogen activities between about 0.16 and 0.21 (11 to percent by volume NH in the binary mixture) and operate at a selected temperature between about 580 and 540 C. as determined from the curve. We have continued this treatment for as long as 440 hours without producing white layer, while obtaining a case depth of up to about 60 mils.

As an example to demonstrate how our invention operates, we employed a two-step method to nitride inch thick sections of a nitriding steel Nitralloy 135M) of the following composition:

Iron Remainder In the first step we used a nitrogen activity of 1.17 (46 percent NH 54 percent H in the binary mixture) at a temperature of 500 C. A 20-hour treatment yielded a case 16 mils in depth and having a surface hardness of 1100 DPH. This steel could be used as a finished product. In the second step we used a nitrogen activity of 0.167 (14 percent NH 86 percent H in the binary mixture) at 'a temperature of 570 C. A 90-hour treatment increased the case depth to 30 mils. Extending the time in the second step to 212 hours, maintaining all other conditions, increased the case depth to 44 mils, still without white layer.

From the foregoing description it is seen that our invention affords a simple effective method of nitriding v and avoiding formation of white layer. Since we never form such a layer, we eliminate costly steps of machining or chemical treatment to remove it. The surface of the case remains as bright as it was before nitriding.

While we have shown and described certain preferred embodiments of our invention, it is apparent that other modifications may arise. Therefore, we do not wish to be limited to the disclosure set forth but only by the scope of the appended claims.

We claim:

1. A method of nitriding the surface of 'a nitriding steel comprising passing over the surface, at substantially atmospheric pressure and at a temperature of about 475 to 530 C., a binary mixture of NH and H having a nitrogen activity of about 0.5 to 1.8 until a case is formed having a hardness of at least 1000 DPH and a depth of at least 16 mils, but terminating this step before nucleating a white layer.

2. A method as defined in claim 1 in which the binary mixture of gases consists of about 30 to 55 percent by volume NH and the remainder substantially H 3. A method as defined in claim -1 in which the nitrogen activity is about 0.6 to 1.8 and the temperature about 500 C.

4. A method as definid in claim 3 in which the binary mixture of gases consists of about 34 to 55 percent by volume NH and the remainder substantially H 5. A method as defined in claim 1 wherein the step defined constitutes the sole nitriding treatment.

6. A method as defined in claim 1 comprising in addition a second nitriding step which consists in passing over the surface, at substantially atmospheric pressure and at a temperature of about 540 to 570 C., a binary mixture of NH and H having an upper limit of nitrogen activity fixed :by the curve shown in FIGURE 2.

References Cited UNITED STATES PATENTS 2,437,249 3/1948 Floe 148166 OTHER REFERENCES Metal Progress, December 1946, The Nitriding of Steel, pp. 1212-1220.

Kohaszati Lapok, 1956, pp. 199-206.

CHARLES N. LOVELL, Primary Examiner.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3 ,399 ,085 August 27 1968 Herbert E. Knechtel et al.

It is certified that error appears in the above identified patent and that said Letters Patent are hereby corrected as shown below:

Column 1, line 52, "pumposes" should read purposes line 66, "stepse" should read steps Column 2, line 31,

should read P 50/ Signed and sealed this 27th day of January 1970.

(SEAL) Attest:

Edward M. Fletcher, Jr.

Attesting Officer Commissioner of Patents WILLIAM E. SCHUYLER, JR.

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