Method of heat treating nickel-base alloys for use as ceramic kiln hardware and product

Disclosed is a NICRALY alloy containing nominally about 16% chromium, about 5.3% aluminum, about 0.02% yttrium and the balance essentially nickel, and heat treated in a manner to develop an essentially aluminum oxide surface. The NICRALY alloy is especially suited for use as components and support systems in kilns that are used in the firing steps in the manufacture of ceramic articles.

This invention relates to nickel-base oxidation resistant alloys, 
particularly to Ni-Cr-Al-Y alloys, and methods of heat treating them for 
use as accessory kiln hardware, components and support systems of kilns 
used in the manufacture of ceramic products. 
Known in the art is a class of superalloy known as NICRALY, these alloys 
contain chromium, aluminum and yttrium in a nickel base. Typical alloys of 
this class are described in many U.S. patents and especially in U.S. Pat. 
No. 3,754,902. 
In the manufacture of typical ceramic products (often called pottery), the 
ceramics, clays, and other non-metallic minerals together with associated 
glazes are usually heated to elevated temperatures three times. The term 
"ceramic products" (and pottery) as used herein includes earthenware, 
porcelain, brick, glass, nitreous enamels and the like products. 
The three firing ranges include: 
1. "Bisque Firing" which removes impurities of nature and which transforms 
the clay mixtures into irreversible chemical compounds. Firing 
temperatures are typically 2100.degree.-2230.degree. F. 
(1150.degree.-1220.degree. C.), 
2. "Glost Firing" during which the glossy glaze layer is fixed to the 
ceramic substrate at temperatures of about 1830.degree.-2010.degree. F. 
(1000.degree.-1100.degree. C.), and 
3. "Decorating Operation" during which decals, colors, hand paintings or 
other decorations are affixed to the pottery. Temperature ranges for these 
operations are typically about 1380.degree.-1830.degree. F. 
(750.degree.-1000.degree. C.). 
Because the in-process ceramic articles are fragile and cannot stand sudden 
extreme changes in temperature without cracking, heating cycles typically 
start at or near ambient temperature, and are slowly raised to the 
required firing temperature. Typical firing cycles are of the order of 
24-48 hours in duration in an oxidizing atmosphere although vacuum or low 
oxygen potential atmospheres could be advantageously utilized. 
During the firing operations, the ceramic articles must be supported so 
that the articles retain the proper shape, while allowing for movement of 
the parts and support system because of thermal expansion, without marring 
the surface finish of the ceramic product. To do this, the alloy may be 
produced in the form of plate, rod or wire and fashioned into various 
support framework devices to hold in-process ceramic objects during the 
firing cycle. Examples of such devices include pedestals, stilts, cradles 
and the like. 
In the present art, these support systems or "kiln hardware" are 
constructed from refractory-type materials into components, which, in 
turn, require preforming and firing to render them serviceable. The term 
"kiln hardware" used herein refers to component parts and support systems 
relating to kilns used in ceramic processing. 
These refractory kiln hardware components have numerous faults, 
shortcomings and disadvantages. They are difficult to make and join, 
costly, friable, brittle and bulky. Further, the present refractory-type 
kiln hardware tends to have a short life, in many instances, only one kiln 
cycle. Furthermore, the ratio of the weight of unsaleable refractory 
support systems to saleable product typically is about 2:1 and frequently 
reaches 3:1. When considering the required energy waste of such systems, 
it becomes imperative to devise and develop more energy efficient methods 
of producing ceramic products. To achieve the required efficiency, support 
systems which can be cycled more rapidly and which have less bulk are 
required. In addition to the energy efficiency required, it is also 
desirable to reduce the tendency of the systems to suddenly crack and 
break (often destroying an entire kiln load of product) or simply break 
during the normal handling of these fragile systems. 
An apparently obvious solution to the above-described difficulties would be 
a metal support system, and this has, indeed, been unsuccessfully tried. 
Stainless steels were tried but, in the long run, the steels lacked 
sufficient strength and oxidation resistance. High temperature 
"superalloys" of the nickel-chromium type, for example 80-20 alloys, 
provided adequate strength levels but left unacceptable discoloration on 
the finished product, because of interaction of the in-process ceramic 
articles and ceramic glaze systems with the naturally forming oxides of 
the alloys investigated. Metal alloys coated with various formulations 
were also investigated. Inconsistent results and poor reliability 
resulted. Thus, what seemed to be an obvious, simple solution to the 
problem of the ceramic industry, in fact proved to be no solution at all.

It is an object of this invention to provide articles particularly suited 
for use as kiln hardware. 
It is another object of this invention to provide a heat treatment method 
that enhances the characteristics of kiln hardware articles. 
Other objects and aims are apparent in the following specification and 
claims. 
The present invention broadly provides a NICRALY alloy article and an 
oxidizing heat treatment to make the article eminently suited for use as 
kiln hardware. 
Through experimentation, it has been discovered that a predominantly 
aluminum oxide scale on an alloy surface is essentially inert to most of 
the raw material mixtures and glazes in the temperature ranges used by the 
ceramic industry. It has been further discovered that alloys of Ni-Cr-Al-Y 
type provide such an aluminum oxide scale when exposed to high 
temperatures, that these scales are essentially self-healing and that the 
scales or oxides are resistant to spalling. 
Finally, it has been discovered that the best results have been achieved 
when the Ni-Cr-Al-Y alloy has been preoxidized at high temperatures to 
preform the insulating- protective-, non-reactive oxide scale prior to 
contact of the surface with the in-process ceramic products to be 
supported. 
A series of heat treatments were performed on a NICRALY alloy to establish 
heating parameters which would adequately form the desired scale interface 
for use between alloy and the in-process ceramic products. 
The alloys used in these tests were comprised essentially of 15% chromium, 
5% aluminum, 0.02% yttrium and the balance nickel. A working range of 
these alloys may vary about 10 to 20% chromium, about 3 to 7% aluminum and 
an effective amount about 0.005 to 0.035% yttrium and balance nickel plus 
impurities and modifying elements, provided the modifying elements do not 
deteriorate the oxide scale that is resistant to discoloration of 
in-process ceramic ware. However, many modifications of the basic NICRALY 
alloy may be made within the ranges 8 to 25% chromium, 2.5 to 8% aluminum, 
a small but effective yttrium content not over 0.04% and the balance 
nickel and impurities plus modifying elements optionally selected from the 
groups: up to 15% total Mo, Rh, Hf, W, Ta, and Cb; up to 0.5% total C, B, 
Mg, Zr and Ca; up to 1% Si; up to 2% Mn; up to 20% Co; up to 5% Ti and up 
to 30% Fe, provided the alloy forms a predominantly aluminum oxide scale. 
The alloys were (1) melted to composition; (2) electroslag remelted (ESR) 
into shapes for further metal working; and, (3) worked into final shape. 
The experimental program to evaluate proper heat treatments resulted in the 
following basic conclusions. 
1. Heat treatment of the subject alloy for one hour at 2100.degree. F. 
provided an adequate oxide film. 
2. The rate of heating to 2100.degree. F. was not critical. 
3. Cold rolling the subject alloy to a reduction of nominally 20% then 
exposing the alloy at 2000.degree. F. for a time of seven (7) hours 
provided an adequate oxide film. 
4. Surface grinding the previously annealed alloy to a 120-grit finish and 
exposing it at 2000.degree. F. for seven hours provided only a marginally 
acceptable oxide film. 
5. Simple exposure of the subject alloy at temperatures below 2000.degree. 
F. did not provide an adequate (a predominantly aluminum oxide) film. At 
these temperatures, a mixture of green (presumably Cr.sub.2 O.sub.3) and 
silver gray (presumably Al.sub.2 O.sub.3) oxides formed. 
6. Exposure of the subject alloy for 20 minutes in flowing argon (a 
simulated bright anneal treatment) created what appeared to be a film of 
Al.sub.2 O.sub.3 but of questionable thickness to provide the desired 
interface. 
7. ESR processed alloy is the preferred method of production. 
From these results, it is concluded that the subject alloy would achieve 
the best surface oxide for interface with ceramic parts during firing by 
being pre-oxidized in an oxygen-bearing atmosphere at a temperature over 
about 2000.degree. F., for example greater than 2100.degree. F., and 
preferably over about 2150.degree. F., but below the melting temperature 
of the alloy for a time dependent upon the condition of the alloy surface, 
the oxygen potential of the atmosphere and the temperature (an exponential 
factor). 
NICRALY alloys may be produced by a variety of processes, powder 
metallurgy, castings, wrought processes and the like as is well known in 
the art. It is preferred, for optimum results, to produce the alloy by the 
electroslag remelting (ESR) process, then hot and/or cold roll to the 
desired article before the critical oxidation step. 
While several methods have been described as a result of testing, other 
modifications may be made within the scope of the invention and within the 
following claims.