US2709145A - Heat-treatment of nickel and nickel containing alloys - Google Patents

Description

United States Patent HEAT-TREATMENT OF NICKEL AND NICKEL CONTAINING ALLOYS Eric Herbert Bncknall, Birmingham, England, assignor tls 'llie International Nickel Company, Inc., New York,

No Drawing. Application November 15, 1951, Serial No. 256,612

Claims priority, application Great Britain November 23, 1950 1 Claim. (Cl. 148-131) This invention relates broadly to the heat treatment of metals and alloys, and more specifically to such heat treatment in which the work is embedded in a metallic packing.

In heat-treating metals or alloys, it is generally required that the work shall not become heavily scaled, and in many cases it is also stipulated that no appreciable change shall be produced in the composition of the metal or alloy layers underlying the surface. For example, in heat-treating steel it is a common practice to define the extent to which decarburisation of the surface is permissible or to what depth detectable decarburisation may occur. In addition to this limitation of the undesirable elfects caused by oxidising or reducing gaseous atmospheres it is sometimes necessary to prohibit pick-up of contaminants such as sulphur during heat-treatment. This is particularly, but not solely, the case when nickel or nickel-containing alloys (including nickel steels) are being heat-treated, since nickel has a strong atfinity for sulphur and the presence of nickel sulphide causes the important mechanical properties of these materials to deteriorate. Sulphur is commonly present to a dangerous extent in the atmospheres of furnaces fired with towns gas or oil and may also be present in electric furnaces in which packing media are employed or special atmospheres are introduced.

It is a common (but by no means uniformly successful) practice to heat-treat metals or alloys while embedded in a packing of metallic turnings or borings, normally of cast iron, within a substantially air-tight container. The packing tends to exclude the air or other furnace atmosphere from the work, and may act in a more positive way in preventing surface attack, e. g., the high carbon content of cast iron gives rise to a slightly carburising atmosphere which is an insurance against decarburisation of steel. In common grey iron turnings and particularly in malleable iron turnings there is, however, often enough sulphur to cause deterioration of nickel steels and similar susceptible materials.

It is an object of this invention to provide an improved heat-treatment process in which the danger of contamination of the work by sulphur is materially reduced.

It is another object of the invention to heat-treat metals or alloys in a special metallic packing.

It is a further object of the invention to heat-treat nickel and nickel-rich alloys in a metallic packing without causing the nickel or nickel-rich alloy to be contaminated by sulphur.

I have discovered that if the work to be heat-treated is embedded in a metallic packing containing a small proportion of magnesium as an alloying ingredient improved results are obtained.

Magnesium is a metal with a high vapour pressure and an extremely high afiinity both for oxygen and sulphur. The high atlinity for sulphur has the consequence that a magnesium-containing alloy made by melting cannot con- Patented Wi ey 24, 19 5573 tain more than a very small percentage of sulphur, because in the course of producing the alloy sulphur present in the raw materials reacts with magnesium to form magnesium sulphide, which separates as a slag.

According to the invention, therefore, I use a magnesium-containing metallic packing, and i find that this is a practical guarantee that the work will be protected from sulphur contamination as Well as from scaling and decarburisation.

The magnesium content of the packing may be very low, and is preferably below 0.5%, but it should be at least 0.02%.

As an example, turnings from cast iron containing 0.1% magnesium as an alloy constituent were used as a heat-treatment packing for a steel containing 8.5% nickel.

Comparison was then made of the mechanical properties of (l) a fully machined test piece of steel treated in this way, (2) a similar test piece of the same steel treated in turnings from ordinary grey iron and (3) an oversized test piece blank of the same steel treated in turnings from ordinary grey iron and subsequently machined to the same dimensions as the other test pieces. In each case, the heat-treatment lasted for 1 hour at 840 C., and at the end of this time the heat-treatment boxes were removed from the furnace and the bars were extracted and quenched immediately in oil. All three bars were tempered at 200 C. for 30 minutes in an oil bath. The mechanical properties were found to be:

It will be seen that the process according to the invention resulted in a combination of properties fully equal to that found in the interior of an oversized blank, where no deterioration could have occurred due to attack during heat-treatment. The fully machined. specimen heattreated in grey iron turnings had much inferior properties, a fact which confirmed that inadequate protection was provided by the ordinary grey iron turnings during heat-treatment.

The merits of the invention have also been shown by heating thin strips of nickel, /20 nickel-chromium alloy, 80/ 15/5 nickel-chromium-iron alloy and 70/30 nickel-copper alloy in the same turnings. After 16 hours at 1000 C., all four strips remained unembrittled and were little discoloured.

I claim:

In the process for heating under substantially nondecarburizing conditions an article made of a metal selected from the group consisting of nickel and nickelcontaining alloys susceptible to contamination by sulfur while said article is embedded in a packing of gray cast iron fragments, the improvement which comprises using as said packing gray cast iron containing magnesium in amounts of about 0.02% to about 0.5% to prevent pickup of sulfur from the surroundings and to prevent excessive scaling and decarburization of said article.

References Cited in the file of this patent UNITED STATES PATENTS 2,124,446 Detwiler July 19, 1938 FOREIGN PATENTS 260,646 Great Britain Nov. 2, 1926 402,609 Great Britain Dec. 7, 1933