Hardsurfaced power-generating turbine components and method of hardsurfacing metal substrates using a buttering layer

A laminated metal structure and a method of hardsurfacing stainless steel base metals are herein provided for resisting wear of steam turbine components at elevated temperatures. The laminated metal structure employs a buttering layer, sandwiched between the stainless steel base metal and a hardsurfacing layer. The buttering layer consists essentially of nickel or a nickel-based alloy and is selected to have a coefficient of thermal expansion which is between that of the base metal and that of the hardsurfacing material. This improved structure produces a relatively crack-free deposit that can provide greater service life for turbine components with less downtime due to repairs.

FIELD OF THE INVENTION 
This invention relates to hardsurfacing metal substrates for high 
temperature service, and in particular, to hardsurfacing turbine 
components to minimize wear. 
BACKGROUND OF THE INVENTION 
Turbine components used in electrical power generation generally encounter 
severe wear due to a variety of mechanisms including: abrasion, erosion, 
fretting, corrosion, and metal-to-metal friction. Especially susceptible 
to this phenomenon are the keys and liners on the steam flow guides 
located on the upper and lower casing of a high pressure turbine. Such 
parts are typically manufactured from stainless steel, i.e., 12% Cr 
material. Since this metal is not hard in its tempered condition (15 to 25 
Rockwell "C"), protective coatings such as hard facings or claddings are 
usually employed to prolong the life of these parts in service. 
One such coating employed is STELLITE "6", which is an extremely hard 
material that has been a standard for hardsurfacing applications. It 
generally produces a surface which resists metal-to-metal wear, abrasion 
and impact. However, Stellite is often associated with cracking due, in 
part, to the differences between the coefficient of thermal expansion 
between the 12% Cr base metal and the welded deposit. Moreover, such 
cracking can also extend into the base metal, which often has been 
hardened intensely from the welding temperatures. This can lead to the 
premature failure of the component and necessitate its repair or 
replacement. 
Replacement of these worn, fretted, or cracked components can be extremely 
costly. Down time alone can amount to $100,000 per day, since the electric 
utility often must buy electrical power elsewhere to meet consumer 
demands. In addition to this cost, the expenses associated with hiring a 
repair crew and purchasing and storing spare parts can be significant. 
In an effort to reduce downtime and the consequent expense, new alloys are 
currently being developed to prolong the service life of turbine 
components. One such alloy is Tribaloy-400 from Cabot Corp of Kokomo, Ind. 
Tribaloy is a cobalt-based alloy and therefore retains its hardness even 
at elevated temperatures. See T. B. Jefferson, et al., Metals and How to 
Weld Them, James F. Lincoln Arc Welding Foundation, Cleveland, OH, 
February 1983, which is hereby incorporated by reference. Deposits of 
Tribaloy and the heat affected zone of the underlying 12% Cr stainless 
steel base metal, unfortunately, have developed cracks and pin holes after 
welding, and therefore, are not completely satisfactory. 
For producing metallurgically sound weld deposits, the welding industry has 
traditionally relied on a "buttering" layer. Buttering has been disclosed 
in the trade literature as a means for applying a transition alloy to a 
base metal that will later be welded to a part of a different chemical 
composition. Birchfield, Part Worn or Undersized? Metal Overlays Save the 
Day, Welding Design and Fabrication, pp. 38-48, February 1985; which is 
hereby incorporated by reference. The Birchfield article reviews various 
processes and materials for the selection of metal overlaying. It 
discloses that buttering provides a metallurgical bridge between different 
alloys and that a buttering material must be readily weldable to the base 
metal and compatible with the joint filler metal that will unite the 
buttering part and mating part. Birchfield, also discloses the following 
examples: high-nickel weld metal deposited on a carbon or low-alloy steel 
substrate, to be welded later to a high-alloy steel base metal; a 
nickel-chromium-iron alloy deposited on a stainless-clad low-alloy steel 
before welding to stainless steel. Although teaching a use for a buttering 
layer, this reference fails to address the problems associated with the 
thermal shock and base metal cracking of turbine components. Moreover, 
this reference requires that the buttering layer be welded to a separate 
buttering part prior to attachment to a base metal mating part. 
Accordingly, there is still a need for a method for hardsurfacing metal 
surfaces to provide turbine components having an extended useful life. 
There is also a need for a repair procedure that minimizes latent welding 
stresses and cracking of stainless steel, on power generation equipment. 
SUMMARY OF THE INVENTION 
A laminated metal structure and a method of hardsurfacing stainless steel 
base metals are provided for resisting wear, preferably at elevated 
temperatures. As used herein, "structure" refers to the novel combination 
of stainless steel base metal, nickel-containing buttering layer, and 
hardsurfacing layer. The novel laminated metal structure employs a 
buttering layer, sandwiched between a base metal and a hardsurfacing 
layer, which acts as buffer between these two materials. The buttering 
layer can be made of nickel or nickel-based alloys and is selected to have 
a coefficient of thermal expansion which is between that of the base metal 
and the hardsurfacing material. Preferably, the buttering layer comprises 
chromium in excess of 12% by weight so that the chromium content of the 
base metal is not diffused out of the base metal during the welding 
process. The laminated metal structure of the invention is substantially 
free of welding-related hot or cold cracks. This improved hardsurfacing 
structure is especially suited for 12% Cr turbine components used in power 
generation equipment. Also disclosed herein is a heat treatment procedure 
for relieving the welding stresses of the weld deposits and for tempering 
the martensite in the heat affected zone of the base metal. The 
combination of hardsurfacing and a buttering layer, especially with the 
post-weld heat treatment of this invention, will provide greater service 
life for turbine components with less downtime due to repairs. 
It is, therefore, an object of this invention to provide a laminated metal 
structure and a method of hardsurfacing that minimizes the shock of 
thermal expansion due to welding procedures. 
It is another object of this invention to provide a laminated metal 
structure and method of hardsurfacing turbine components that utilizes a 
buttering layer with a coefficient of thermal expansion intermediate to 
that of the base metal and the hardsurfacing material. 
It is still another object of this invention to provide a laminated metal 
structure and method of hardsurfacing that extends the useful service life 
of turbine components. 
It is still another object of this invention to provide a laminated metal 
structure and hardsurfacing procedure for turbine components that 
minimizes power plant downtime and spare part inventories. 
It is still another object of this invention to prevent the diffusion of 
chromium from the base metal during the application of a hardsurfacing 
deposit. 
With these and other objects in view, which will become apparent to one 
skilled in the art as the description proceeds, this invention resides in 
the novel construction, combination, arrangement of parts and methods 
substantially as hereinafter described and more particularly defined by 
the attached claims.

DETAILED DESCRIPTION OF THE INVENTION 
The present invention provides a laminated metal structure that is 
wear-resistant, preferably at elevated temperatures, and a method of 
hardsurfacing to provide the same. The procedure and structure herein 
described may be employed in any high temperature service application, for 
example, heat treating furnaces, internal combustion engines, and valve 
applications. However, this invention is particularly useful for turbine 
components, more particularly those made from 12% Cr stainless steel, 
which undergo heavy oxidation and scoring in steam filled chambers. The 
structure and method are designed to protect the relatively soft stainless 
steel base metal, which is often subjected to metal-to-metal friction at 
temperatures above about 900.degree. F. The structure incorporates a 
buttering layer interposed between the base metal and hardsurfacing layer. 
The buttering layer is composed of nickel, nickel-based alloys, or a 
mixture of the two. These materials are chosen for the buttering layer 
because nickel and its alloys have coefficients of thermal expansion which 
lie between that for the stainless steel base layer and that of most 
hardsurfacing materials commonly used in turbine applications. The 
hardsurfacing layer, which is superposed on the buttering layer of this 
invention, preferably comprises cobalt to maintain the high temperature 
hardness of the laminated metal structure under power-generating service 
conditions. 
With reference to the FIGURE, there is shown a laminated metal structure 
100 which is designed for resisting wear, especially at elevated 
temperatures. The structure of the FIGURE can represent, for example, a 
section of a turbine component such as a key or liner. The structure 100 
comprises a base metal 10 comprising stainless steel. Bonded to the base 
metal 10, is a buttering layer 20 which consists essentially of nickel, a 
nickel-based alloy or a mixture thereof. Superposed upon the buttering 
layer is a hardsurfacing layer 30 of commonly used hardsurfacing material, 
but which preferably comprises cobalt for improved high temperature 
hardness. The combination of the buttering layer 20 and the hardsurfacing 
layer 30 of this invention form a substantially continuous coating for 
protecting the base metal 10 from wear at elevated temperatures. As used 
herein, "coating" refers to any mechanical or metallurgical bonding of 
metal to the base metal 10, and includes deposits produced by cladding, 
welding or thermal spraying. 
The base metal 10 of this invention is preferably chromium stainless steel 
containing about 8 to 16% chromium. Generally, this material refers to a 
group of stainless steels that contain no nickel. This group is frequently 
called "straight chromes" or martensitic stainless steels. However, the 
martensitic nature of the steel will greatly depend on the carbon content, 
a high carbon content tending to make the steel more martensitic. Such 
steels are generally assumed to be heat treatable and comprise the 
stainless steel specification numbers AISI, 403, AISI 410, AISI 414, AISI 
416, AISI 418 Special, 420, 420 Se, 431, AISI 440A, AISI 440B, AISI 440C, 
and AISI 440Se. The basic type used in the manufacture of turbine 
components, i.e. keys and liners, is AISI 410 or AISI 403, each of which 
has a chromium content of about 12%. The materials of this series, because 
of their alloy balance, are capable of hardening intensely from welding 
temperatures, even with an air cool, and unless precautions are taken, 
they (and the weld materials used with them) can crack because of the high 
hardness developed. Preheating the steels, however, can lower thermal 
differences, and allowing the steel to cool slowly will reduce the 
cracking tendencies. It is important to the purposes of this invention 
that the base metal and welded structure remain as crack-free as possible 
to avoid failure. 
Although a straight 12% Cr material such as AISI 403 or AISI 410 is 
preferred for use as the base material of this invention, the 
hardsurfacing technique is also useful on mild steel, alloyed steel, high 
carbon steel, and/or a combination of these. However, since the turbine 
power generation environment is corrosive, the preferred base metal 10 of 
this laminated metal structure is stainless steel, and more preferably 12% 
Cr stainless steel. 
The buttering layer 20 of this invention in a soft material having a 
thermal expansion coefficient between that of the base metal and that of 
the hardsurfacing layer. Preferably, the buttering layer consists 
essentially of nickel, a nickel-based alloy, or a mixture of these. The 
buttering layer 20 is bonded, preferably welded, to the base metal 10. The 
buttering layer 20 also adds to the overall strength of the structure, 
since the bond between the hardsurfacing layer 30 and the base metal 10 
may not be strong enough by itself to keep a multi-layer hardsurfacing 
deposit from pulling off. See T. B. Jefferson, et. al., Metals and How to 
Weld Them, pp. 297, 298. 
The buttering layer of this invention is selected to be a relatively soft 
material to withstand the shock of the thermal expansion caused by the 
heat of the welding procedures. Nickel and its alloys are as the preferred 
metals for the buttering layer 20 of this invention because the 
coefficient of thermal expansion of these materials is in-between that of 
the preferred 12% Cr base metal and the preferred hardsurfacing materials. 
Nickel and nickel alloys are additionally preferred because of their 
excellent resistance to corrosion and oxidation even at high temperatures 
and because they permit an overlay of the preferred hardsurfacing 
materials without significant cracking. Additionally, the buttering layer 
20 should also contain above about 12% chromium by weight. This prevents 
the diffusion of chromium from the 12% Cr stainless steel into the weld 
deposit during welding and preserves the corrosion resistance of the base 
metal 10. 
One important alloy that can be used for this purpose is Monel, which is 
67% nickel, 28% copper and 5% manganese and silicon combined. Monel is 
especially valuable where the turbine components are subject to wear and 
corrosion. Also useful are the heat-hardened variations of Monel such as, 
K Monel, H Monel, S Monel, R Monel and N Monel. The most preferred 
material for the buttering layer 20 is Inconel which is a nickel-chromium 
alloy. Of particular interest to the purposes of this invention, is the 
material Inconel-82, ASME SF 5.14, class ERNICR-3, which material exhibits 
the following composition by weight percent: C (0.1); Mn (2.5-3.5); Fe 
(3.0); P max (0.03); S max (0.05); Si (0.5); Cu max (0.5); Ni min (67.0); 
Co max (0.1); columbium and tantalum (2.0-3.0); Ti max (0.75); tantalum 
(0.3); and Cr (18.0-22.0). The buttering layer 20 generally has a 
thickness of about 0.25 mm to 5.0 mm, more preferably about 2 mm. This 
thickness can be attained by machining the layer after bonding to the base 
metal. 
The material selected for the hardsurfacing layer 30 should have a Rockwell 
hardness reading significantly higher than that for the base metal, 
preferably in the range of 30-55 R.sub.c. In general, the choice of 
hardsurfacing materials can be made on the basis of service requirements 
and the nature of wear and other conditions that are expected. Typical 
hardsurfacing alloys useful for this invention include chromium carbide, 
tungsten carbide, high-carbon chromium alloy, austenitic manganese, 
austenitic stainless steel (chromium-nickel types), high-speed tool 
steels, air or oil hardening tool steels, medium carbon and alloy steels, 
cobalt-based hardsurfacing materials and nickel-based hardsurfacing 
alloys. The hardsurfacing alloys of this invention preferably comprise 
cobalt to impart to the finished turbine component high hardness at 
elevated temperatures. The most preferred materials are Tribaloy-400 and 
Stellite-6, the former being selected as a preferred material for 
stationary turbine components and the latter being preferred for moving 
turbine components. While Stellite-6 is conventional, Tribaloy-400 is a 
relatively new alloy consisting essentially of 0.02 weight %C; 2.6 weight 
%Si; 8.5 weight %Cr; 28.5 weight %Mo.; 3.0 weight %Ni & Fe; and the 
balance being Co. These materials have been selected for example only, and 
those skilled in the art may find various alloy substitutions exhibiting 
similar properties. The hardsurfacing layer of this invention preferably 
has a thickness of about 1.27 to 6.35 mm, and more preferably about 4.57 
mm. 
The buttering layer 20 and hardsurfacing layer 30 can be bonded to or 
disposed on the base metal 10 by any mechanical, metallurgical, or 
chemical means known to those in the metal-working industry. Preferably, 
the buttering layer 20 is weld deposited to the base metal layer and the 
hardsurfacing layer 30, in turn, is weld deposited to the buttering layer. 
Typical coating processes that can be employed for this purpose include: 
Thermal Spraying, Plasma Transferred-Arc, Shielded Metal Arc, MIG, or TIG 
procedures. The most preferred welding process for the purposes of this 
invention is TIG (Tungsten-Inert-Gas). Below, is a detailed summary of the 
preferred basic parameters for applying the preferred buttering layer 20, 
Inconel-82, and the preferred hardsurfacing layer, Tribaloy-400, using a 
TIG welding process: 
______________________________________ 
BASIC AMETERS 
INCONEL-82 
TRIBALOY-400 
______________________________________ 
AMPS 150-180 200-225 
VOLTS 20-25 25-30 
GAS FLOW 15 CFM 15 CFM 
FILLER WIRE 3/32" 3/16" 
DIAMETER 
SHIELDING GAS Argon 99.8% Argon 99.8% 
ELECTRODE - 2% thor. 2% thor. 
TUNGSTEN 
______________________________________ 
In a preferred method of this invention, Inconel-82 is welded to the base 
metal, using the above procedure, to provide a buttering layer thickness 
of about 2.54 mm. The buttering layer is preferably machined to a 
thickness of about 2.03 mm. The resultant intermediate composite, which 
comprises the base metal and buttering layer, is then heated to a 
temperature of about 232.degree. C. to 482.degree. C., preferably about 
400.degree. C., prior to and during the welding of the hardsurfacing layer 
30 onto buttering layer 20. As used herein, the temperature of the 
substrate during the welding of the hardsurfacing layer 30 is referred to 
as the "interpass temperature". 
Preheating is useful to (1) minimize the possibility of thermal shock 
damage to the stainless steel as the welding arc is applied; (2) slow the 
cooling to prevent the formation of excessively large fusion zones; (3) 
prevent excessive hardness in the weld itself; and (4) equalize the 
cooling of the weld and the base metal, thereby minimizing the possibility 
of shrinkage cracks. See T. B. Jefferson, et. al., Metals and How to Weld 
Them, p. 332. 
After depositing the hardsurfacing material, the laminated metal structure 
is preferably tempered with a post-weld heat treatment. When the base 
metal 10 is the preferred 12% Cr stainless steel, the temperature selected 
for the post-weld heat treatment should be high enough to temper 
martensite formations in this base metal. The preferred heat treatment 
schedule comprises heating the welded structure for a time that is 
sufficient to relieve at least the maximum stresses caused by the various 
bonding steps of the method of this invention. A preferred heat treatment 
range for this invention is about 593.degree. C. to 627.degree. C. 
The laminated metal structure 100 resulting from the bonding operation, as 
described, is generally at a temperature of about 232.degree.-482.degree. 
C. The structure can be permitted to cool in an insulating atmosphere, or 
maintained at about its interpass temperature prior to insertion into a 
heat-treating oven. The laminated metal structure 100 is then heated at 
about 30.degree.-40.degree. C. per hour, preferably about 38.degree. 
C./hour, until it obtains a uniform temperature of about 593.degree. C. to 
627.degree. C. The structure 100 is then held at that temperature for 
about 1 to 3 hours. Preferably, the structure is held at the optimum 
temperature for one hour per inch of thickness of the component, with one 
hour being the minimum holding time for pieces under one inch in 
thickness. After heat treatment, the component is preferably allowed to 
cool at a rate below about 38.degree. C. per hour. This can be 
accomplished by cooling in an insulating environment, i.e. Vermiculite, 
Kaowool or a slow furnace rate cooling. This cooling step is continued at 
least until the laminated metal structure 100 obtains a uniform 
temperature of about 149.degree. C., at which point the cooling rate is 
less important and the structure can be exposed to ambient temperatures. 
It must be noted that this heat treatment schedule is for illustrative 
purposes only, and those skilled in the art may find alternative heat 
treatment schedules that will provide sufficient tempering for both the 
weldment and the heat-affected zone of the base metal. 
In summary, this invention provides a laminated metal structure 100 
exhibiting fewer imperfections, while at the same time, maintaining 
resistance to wear from room temperature to elevated temperatures. The 
preferred combination of Inconel-82 and Tribaloy-400 produces a surface 
having a Rockwell "C" hardness reading of about 40-50. The Inconel-82 
buttering layer possesses a coefficient of thermal expansion intermediate 
to that of the base metal and the preferred hard surfacing material, 
Tribaloy-400. Inconel-82 also consists of 18.0 to 22.0 weight percent Cr 
which permits the deposition of this alloy with minimal dillution of the 
chromium content of the stainless steel base metal. Finally, the structure 
of this invention is readily fabricated using existing welding procedures 
and provides for longer lasting turbine components. 
This structure and method are particularly useful in minimizing cracking 
and the service life of stainless steel keys and liners located on the 
flow guides of steam turbine systems. These components can be coated with 
a nickel-containing, buttering layer, of the kind described above, then 
hardsurfaced. Accordingly, the elevated temperature turbine system, thus 
protected, can be operated with less donwtime due to replacement of these 
critical parts. 
From the foregoing, it can be realized that this invention provides an 
improved laminated metal structure and method for resisting wear or 
elevated temperatures. The laminated metal structure utilizes a novel 
buttering layer which provides a buffer and limits the thermal shock 
caused by the welding processes. Accordingly, this invention provides a 
sounder hardsurfacing weldment and specifically, a more economical 
TRIBALOY application procedure without a high rejection rate due to 
cracking of weld deposits. Although various embodiments have been 
illustrated, this was for the purpose of describing, not limiting the 
invention. Various modifications, which will become apparent to one 
skilled in the art, are within the scope of this invention.