US3778299A - Rotary barrel diffusion coating in molten lead - Google Patents

Abstract

A NOVEL PROCESS FOR DIFFUSION COATING OF FERROUS ARTICLES IN A LEAD BATH BY SEALING ALL OF THE CONSTITUENTS IN A BARREL WHICH IS ROTATED IN A NON-VERTICAL POSITION AT THE CHROMIZING TEMPERATURE. SUCH PROCESSING RESULTS IN EXTREMELY DIFFUSED ZONES ON IRREGULARLY-SHAPED PARTS.

Description

United sates pat nt once 3,778,299 ROTARY BARREL DIFFUSION COATING MOLTEN LEAD 1 John J. Rausch, Antioch, and Ray J. Van Thyne, Oak Fawn, 111., assignors to Surfalloy Corporation, Chicago,

l]. 1 Drawing. Continuation-impart of application Ser. No. 768,187, Oct. 16, 1968, now Patent No. 3,620,816.

This application Nov. 12, 1971, Ser. No. 198,404

- Int. Cl. C23c 9/00 US. Cl. 117-109 3 Claims ABSTRACT on THE DISCLOSURE A novel process for diffusion coating of ferrous articles in a lead bath by sealing all of the constituents in a barrel which is rotated in a non-vertical position at the chromizing temperature. Such processing results in extremely diffused zones on irregularly-shaped parts.

CROSS REFERENCE TO RELATED APPLICATION This application is a continuation-in-part of our copending application Ser. No. 768,187 entitled, Method of Diffusion Coating Metal Substrates Using Molten Lead as Transport Medium, filed Oct. 16, 1968 now US. Pat. 3,620,816.

BACKGROUND OF THE INVENTION In our copending US. patent application (Ser. No. 768,187) now U.S. Pat. 3,620,816 there is described a diffusion coating process in which a metal article is surface alloyed by contacting the article with molten lead containing one or more diffusing elements. This process particularly provides chromium impregnated (chromized) ferrous articles which have good resistance to corrosion. In general, the process involves immersing the article to be coated in a molten lead bath for a predetermined time, removing the diffusion coated article from the bath, and thereafter cleaning the article to remove adherent lead and bath residue. In order for the process to be carried out more effectively we have found that some degree of agitation of the parts and the bath must occur. It is desirable that the elements that are being transferred to the ferrous article to be maintained at some effective degree of solution concentration in the lead. It is also desirable that the ferrous articles not remain in contact with each other for prolonged time periods since the lead solution containing the transfer elements can not effectively maintain saturation and cause transfer under such conditions. Furthermore, said ferrous articles .will tend to bond together if allowed to reside in contact for prolonged time periods in the bath. It should also be pointed out that ferrous articles will float in a lead". bath and in order for diffusion coating to occur such articles must be maintained below the surface of the bath at least during most of the diffusion coating cycle. We have also found that it is desirable to exclude air or other gases that will react with the lead, chromium, or other ingredients of the bath. Either vacuum or an inert gas such as argon, for example, are suitable covers for the bath. However, if the contained space above the bath is large the impurities present in the atmosphere can cause significant contamination of the bath. Furthermore, if that space is large a significant amount of lead is lost by evaporation. Thus it is most desirable to have the bath sealed with only a small residual space such that very little contamination or evaporation can occur. If this space is small enough even air can be left in the space since it will not contaminate the bath significantly.

We have discovered an effective means of diffusion coating with our lead bath process that eliminates all of the problems heretofore described. In this method, all of 3,778,299 Patented Dec. 11, 1 973 DESCRIPTION OF THE INVENTION When the process is used for chromizing, the ferrous parts, chromium source and other diffusing elements are charged into the barrel which is heated to a temperature in the range 600 to 800 F. The barrel is continuously purged with argon prior to and during the filling cycle to avoid oxidation of the materials. Molten lead is poured into the barrel until about of the volume is filled With lead and the other materials. The barrel is then sealed by welding or it is preheated, while the inside is either evacuated or purged with an inert cover gas, to a temperature near the chromizing temperature (-1950 F.) and then sealed with an external flange. The flange is normally located sufficiently far from the vaned barrel (the reaction chamber) such that a gasket, Teflon, for example, can be used for sealing. The external flange is connected to an insulated plug having a diameter slightly less the ID. of the barrel. If the plug is of sufficient length such that the flange is below the melting point of lead it is possible to use solid lead, which will form when the barrel is placed in a horizontal position, as a sealant.

The barrel can be made of iron, or an alloy of iron that contains the diffusing elements being transferred, provided those elements are not extensively soluble in lead. For chromizing, we frequently use Fe-Cr stainless steels such as A.I.S.I. type 446 (Fe-25 Cr). Under these conditions the chromium in the alloy will not be dissolved significantly since the chromium source in the bath will have a higher chromium content. Nickel-base alloys cannot be used, without an intermediate liner, since nickel is readily dissolved by lead.

For 4 in. ID. barrels we normally use 4 vanes, l in. long which are attached radially and spaced 90 apart. Barrels of this type have been rotated at speeds ranging from k to 6 rpm. and have resulted in very uniform chromizing of all of the parts in the barrel. For larger diameter barrels more numerous and longer vanes could be used to accomplish effective bath stirring and relative motion of the parts.

The reliability of our rotating barrel process has been demonstrated by many examples. Over 35 separate runs have been made using ferrous parts with internal holes and threads. Typically the chromizing treatments were 1950 F. for 4 hours, which produces a chromized layer about 0.003 inch deep on very low carbon-containing parts. Of course other treatments may be used depending upon the desired chromizing zone depth. Over parts that were selected at random have been sectioned and metallographic examination at 100x magnification showed that in every example the chromized layer was uniform.

We have successfully used both ferrochromium and un-alloyed chromium. In addition to chromium per se, the process can be used to diffuse other elements along with chromium or to diffuse other elements, soluble in lead, in the absence of chromium. Examples of the latter include aluminum, titanium, silicon, cobalt, nickel, and others. Our barrels are normally operated in a horizontal position but they could be operated in other non-vertical positions, up to 60 degrees from the horizontal, with suitable vaning designs.

We claim as our invention:

1. A method of diffusion coating ferrous articles using a lead bath containing a source of the diffusing elements comprising the steps of (a) providing a molten lead bath containing the elementor elements to be diffused;

(b) placing said bath and said ferrous articles in a sealable, vaned cylindrical barrel; and filling said barrel therewith to minimize free space over said bath;

(c) sealing said barrel;

(d) rotating said barrel in a position ranging from horizontal to 60 degrees from the horizontal While maintaining the bath therein at a molten temperature and while rotating maintaining said ferrous articles substantially submerged within said bath; and

(e) maintaining said rotation for an adequate time to diffusion treat said ferrous articles.

2. A method as defined in claim 1 in which the difiusing element is chromium.

3. A method as defined in claim 1 wherein the barrel is held in and rotated in a horizontal position.

References Cited UNITED STATES PATENTS 3,377,195 4/1968 Sneesby 117114 R 2,793,965 5/1957 Myers et al. 117109 2,864,731 12/1958 Gurinsky et al. 1486.l1 2,929,741 3/1960 Steinberg 117-118 X 135,339 1/1873 Hunt 117-109 X 1,713,233 5/ 1929 Kelley 1171 14 BX 2,910,379 10/1959 Gurinsky 117118 X 2,926,111 2/ 1960 Schweitzer et al. 1486.11

15 RALPH s. KENDALL, Primary Examiner US. 01. X.R. 117-414 R, 119