FIELD OF INVENTION
This invention relates to a process of producing a hard face on a very thin but
large diameter stainless steel vessel maintaining stringent dimensional
requirement on the hard faced ID section. More particularly, the invention
relates to a process of depositing a hard faced material on the thin walled
stainless steel (SS) vessel inner surface effecting a defect free uniform hard
coating by using fixturing and controlled rate of heating and cooling during spray
deposition and subsequent heat treatment.
BACKGROUND OF THE INVENTION
A thin vessel is always subjected to wear during operation. Hence in order to
avoid this wearing, a need is felt to eliminate the disadvantage.
A thin vessel made up of stainless steels (SS) is now coated with Nickel-Base
Hard-Surfacing Alloy (hard face coating) to resist the wear during operation. The
Vessel diameter to thickness ratio is around 185 and the inside surface is
provided with hard-face coating with min thickness of 1mm. After hard face
coating the vessel is subjected to heat treatment and then the inner surface of
the vessel is machined to obtain a true inner diameter with stringent dimensional
tolerance. Inside the vessel a piston like shell assembly will be moving up and
down due to the temperature gradient exists around this shell assembly.

Method of depositing the hard faced material on the stainless steel vessel inner
surface is carried out by Plasma Transferred Arc Welding (PTAW) process, which
is carried out at relatively high temperature. Plasma transferred arc welding
(PTAW) is a high energy-intensity process which is used to deposit a thick layer
of a wear resistant alloy. The hard face coating material melts at a temperature
around 1050 deg C. For effective bonding of hard face material to Stainless steel
vessel/component has to be heated to temperature around 650 deg C. Factors
that have a strong effect on the weld deposit are the weld speed, carrier
gas/powder speed, and arc current, while other variables such as powder and
gas composition and the parent material temperature.
The Stainless steel and the hard facing material has different thermal coefficient
of expansion (values are listed in the table -1). Process-induced thermal gradient
and large difference in the thermal coefficient of expansion exists between the
deposit and heated base metal generates thermal stresses. Residual stresses due
to thermal gradient are known to affect the performance of hard faced
components. The magnitude and distribution of the thermal stresses vary
depending on the preheat temperature, coating thickness, heat input, deposition
process, and the geometry.

During cooling the SS vessel will shrink more due to its higher coefficient of
thermal expansion than the coated hard faced material. Higher magnitude of
temperature gradient during heating and cooling may severely affect the coating
properties and may cause severe cracks on the coated surface.
On successful completion of hard face coating the assembly is subjected to heat
treatment without reducing the coating temperature to remove thermal stresses.
External heating coils is used for preheating and post heat treatment of SS vessel
assembly.
OBJECTS OF THE INVENTION
It is therefore an object of the invention to propose a process of producing a
hard face on a very thin but large diameter stainless steel vessel maintaining
stringent dimensional requirement which is capable of producing defect free
coated surface of stainless steel vessel.
Another object of the invention is to propose a process of producing a hard face
on a very thin but large diameter stainless steel vessel maintaining stringent
dimensional requirement which ensures a crack free uniform coating on a large
surface area of stainless steel vessel.

A further object of the invention is to propose a process of producing a hard face
on a very thin but large diameter stainless steel vessel maintaining stringent
dimensional requirement, which results effective bonding of hard faced material
to the stainless steel material to the stainless steel material by controlling
heating and heating rate and thus ensuring uniform enlarging of vessel.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
The invention can now be explained with reference to the accompanying
drawings where
Figure 1 is a schematic arrangement which shows preparation of large size
stainless steel vessel to be coated inner surface with Nickel-Base
Hard-Surfacing Alloy.
Figure 2 is a sectional view of vessel with stiffener arrangement according to
this invention.
Figure 3 is schematic diagram for preparation of internal surface by groove
machining for coating.

with thermal insulation.
Figure 5 is a top view of vessel showing location of PTAW torch along with
heating coils and insulation.
Figure 6 final coated and machined vessel assembly.
DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT
OF THE INVENTION
The inventive process ensures a defect free coating and maintaining the
stringent circularity on a very thin stainless steel vessel
The following problems are faced to establish a defect free coating on a very thin
stainless steel vessel. They are:
(i) Unable to retain the cylindrical shape of very thin stainless steel vessel,
(ii) Higher order uneven shrinkage of vessel dimension (vessel shape is
changed)
(iii) Severe surface cracks on the coated material,
(iv) Because of change in the shape, the vessel is unable for machining,
(v) To control the vessel ovality initially vertical stiffeners were used. Even
with provision of vertical stiffener, the outer vessel surface is unable to
retain the vessel shape.

The following improvements are implemented in the present invention.
Additional circular stiffener rings are welded over the vertical stiffeners to provide
additional rigidity to the vessel and retain reasonable ovality / circularity. The
ovality / circularity problem is overcome by welding of temporary circular thick
stiffening ring (5) over the vertical stiffeners (4) at three elevations, as shown in
Figure 2.
After welding stiffeners additional step machining (Fig-3) between the grooves
are provided which is used for finer profile correction before the coated vessel is
subjected for ID boring. This arrangement is helped elimination of vertical cracks
For effective bonding of hard faced material to the stainless steel material, the
stainless steel vessel is heated and retain at very high temperature (Pre heat
temperature 650 deg C) during coating process. At the end coating process the
vessel is to be heat treated with reduced temperature. For heating and cooling
of the vessel heating coils are surfaced as shown in Fig-4 with appropriate
controllers. Similarly internal surface of the vessel is also provided with heating
coil with appropriate opening for housing the PTAW torch. Over the heating coils
provided with insulation material. The outer surface of the vessel is provided with
thermocouples at different elevation. Current to the heating coils are controlled
using temperature controllers.

After reaching the right temperature, hard surfacing of alloy material is
performed by plasma transferred arc welding (PTAW) process. During the
coating process the rise of the vessel material temperature is controlled by
varying the current in the external/Internal heating coils. On successful
deposition of required thickness of coating of hard face material the entire
assembly is continued for heat treatment for stress relieving using the external
heating coils.
The formation of crack on the hard face surface is occurred mainly due to higher
rate of heating (110-150 deg C/hr.) / cooling (100 deg C) which results in
differential expansion / contraction (app 0.6 to 0.9mm/100 deg drop) of stainless
steel and hard facing material. The rate of contraction is very high at higher
temperature. To reduce the differential expansion/ contraction upto 300 deg C,
controlled rate (10 to 15 deg /hr) of heating and cooling is established in such a
way that the differential temperature of vessel material and coating material is
kept very minimum. This is achieved by controlling heating system. This has
resulted in successfully developed crack free uniform coating on a large surface
area.

Stainless steel material along with coating contracts radially by 4 to 5 mm for the
specified vessel size. The vessel radial shrinkage problem is overcome by
increasing the internal diameter by 4 mm before coating process is effected.
During hard face coating the stainless steel cylindersity is bound to change and
its profile will become marginally oval. Before ID boring the vessel profile is
corrected to nearest to circular profile using Lug spider. Vessel correction is
performed after removal of all the stiffeners.
Finally the vessel assembly is subjected to internal diameter boring to establish
the required control dimension.
The set up for hard surfacing alloy coating on a large shape vessel as shown in
Figure 1, has the following components.
First the vessel (1) is formed by rolling using a higher thick material to take care
of rolling ovality and welding shrinkage. After rolling, long seam welding and
associated components (Flange (2) and cone (3)) are welded to the rolled shell
to form vessel assembly.

The vessel assembly is welded with 12 numbers of vertical stiffener (4), 3
numbers of circular stiffener rings (5) and 16 numbers of angular stiffeners (6).
The stiffener welding arrangement is shown in Figure-2. The material and sizing
of the stiffener is selected to provide stiffness to vessel assembly during hard
facing process.
Vessel heating/cooling arrangement is shown in Fiqure-4 and 5. Heating coils (7)
wound around the outer and inner surface of the vessel is used for
heating/cooling and maintenance of temperature during hard facing process. The
entire heating arrangement is thermally insulated (8). At the inner heating
arrangement a rectangular opening is provided to house the PTAW torch (10).
The entire arrangement is mounted on rotating flat form or rotor head (9).
During spray process the vessel assembly is rotated uniformly. Spray head is
mounted on a axial tracers arrangement.
PTAW hard facing process is performed on the inner surface of the vessel.
During hard facing vessel temperature is closely monitored / controlled by
dynamically varying the heating current.
On successful hard facing process, the stiffeners are removed and the assembly
is corrected for ovality within 1 mm. The assembly is then machined for inner
surface.

The following are the broad physical parameter of component which are
considered in the invention.
vessel diameter - 2200 mm
vessel thickness - 12 mm (min)
vessel height - 550 mm
vessel material - stainless steel (SS)
coating material - nickel-base hard surfacing alloy
volume of material deposited - ~100 kg
average co-efficient thermal
expansion of stainless steel - 19.9 µm / mk
thermal conductivity at 500° C - 21.4 w / mk
average coefficient of thermal
expansion of hard facing material - 14-15 µm / mk
vessel bottom is provided with inside flange
vessel top section is provided with outward cone

WE CLAIM
1. A process of producing a hard face on a very thin but large diameter
stainless steel vessel maintaining stringent dimensional requirement,
comprising:
- heating the stainless steel vessel to a very high temperature and at a very
low temperature rate to have effective bonding of hard faced material to
the stainless steel material and also to ensure uniform enlarging of vessel;
- achieving the right temperature, coating of hard surfaced alloy material
effected by plasma transferred arc welding (PTAW) process;
- varying the current in the external/internal heating coils to control the rise
of the vessel material temperature during the coating process;
- at the end of coating process the vessel is continued for heat treatment
(stress relieve) by controlling the external heating element;

- controlling rate of heating and cooling by controlling external/internal
heating system to keep the differential temperature of vessel material and
coating material minimum to develop crack free uniform coating on a
large surface area;
- increasing the internal diameter of the vessel before coating process
effected to account for the vessel shrinkage problem;
- overcoming the problem of circularity by welding temporarily circular thick
stiffening ring over the vertical stiffeners at three elevations, and
- after successful hard facing process, the stiffeners are removed and the
assembly is corrected for ovality within 1 mm,
- and the assembly is finally machined for inner surface.

2. The process as claimed in claim 1, wherein the coating material is nickel-
base hard surfacing alloy.
3. The process as claimed in claim 1, wherein heating coil is wound around
the outer/inner surface of the vessel to heat/cool and maintain
temperature during hard facing process.
4. The process as claimed in claim 1, wherein the stainless steel vessel is
heated to a temperature around 650° C to make the bonding of hard face
material effective.
5. The process as claimed in claims 1 to 4, wherein the control rate of
heating and cooling is 10 to 15 degree per hour that keeps the differential
temperature of vessel material and coating material at maximum.
6. The process as claimed in claims 1 to 5, wherein the internal diameter of
the vessel is increased by 4 mm before coating process is effected to
overcome the vessel radial shrinkage problem.

7. The process as claimed in claims 1 to 6, wherein the vessel assembly is
welded with 12 numbers of vertical stiffener (4), 3 numbers of circular
stiffener rings (5) and 16 numbers of angular stiffeners (6) to provide
additional rigidity to the vessel.

The a process of producing a hard face on a very thin but large diameter
stainless steel vessel maintaining stringent dimensional requirement on the hard
faced ID section. Plasma transferred arc welding (PTAW) process is employed for
depositing the hard face material on to the SS vessel. For effective bonding of
hard face material to the SS vessel, the vessel assembly is heated to a very high
temperature. Immediately after coating the vessel assembly is heat treated to
remove the thermal stresses.
The large difference in coefficient of expansion of SS vessel assembly and coated
material concerns the production of defect free and distortion in the vessel
shape. Suggested rate of heating and cooling employed in the coating process
resulted in vessel is heavily deviated and also crack is generated on the entire
surface of coated section.
The process of production of defect free on the coated material is achieved by
controlling and lowering the rate of heating and cooling during preheating,
PTAW, heat treatment and cooling process. Lower rate of heating /cooling using
temperature controllers resulted in lowering rate of differential
expansion/contraction of vessel assembly and the hard faced coated material
favored the defect free coating.
Marginal increase of internal diameter of the vessel before coating process
resulted in achieving the desired final dimension after coating and final
machining. Vessel shrinkage and retaining the circularity problem is overcome by
welding temporary circular thick stiffening ring over the vertical stiffeners at
three elevations. After coating these stiffeners are removed and the assembly is
corrected for ovality within 1 mm and the assembly is finally machined for inner
surface.