Claims:We claim:
1. A method for post weld heat treatment of Inconel 617 pipe, the method comprising of steps:
calculating (101) Heat Band (HB) of a pipe (1);
calculating (102) number of heating elements (3) required to be wound over the calculated Heat Band of the pipe (1);
winding (106) the calculated heating elements (3) on the Heat Band of the pipe (6); and
supplying (108) power to the heat elements (3) from a controlled power source (6) to heat up the Heat Band of the pipe (1) upto a threshold temperature.

2. The method as claimed in claim 1, wherein the Heat Band is calculated by equation (1):
HB = (t + W + t) + [4 * (R1*t)1/2]
Where, ‘t’ – Thickness of weld, ‘W’ – Width of widest weld segment on the outer surface of the pipe, “R1’ – Outer Radius of the pipe.
3. The method as claimed in claim 1, wherein the heating element (3) are ceramic pad heating element (CP48).
4. The method as claimed in claim 1, wherein the calculating (102) number of heating elements further comprises:
calculating (103) mass of material of the Heat Band (HB) to be heated using equation (2):
(M) = ? * (3.1416) * HB * (R12 – R02)
Where,
‘?’ – Density of the material Inconel 617
‘R0’– Inner radius of the pipe
calculating (104) energy required for heating the calculated mass of the material using equation (3):
(E) = M * Cp * (980 – AT)
Where,
‘T’= (980-AT)/(220/t)
calculating (105) power required for heating the calculated mass of the material using equation (5):
P = (1.2 * E) / T
And N (number of heating elements) = P/2.9.

5. The method as claimed in claim 1, wherein the method further comprising:
sensing (109) temperature of the Heat Band by a thermocouple (2) placed at centre of the welded pipe (1) and sending feedback signal having information about temperature of the Heat Band to the power source (6).
6. The method as claimed in claim 6, wherein the method further comprising:
maintaining (110) temperature of the Heat Band upto a threshold limit using sensed temperature of the Heat Band.
7. The method as claimed in claim 1, wherein the method further comprises:
wrapping (104) of insulation material around the heating elements (3) and adjoining areas to avoid loss of convective heat.
8. The method as claimed in claim 8, wherein ‘N’ number of heating pads are wound helically over the outer surface of welded zone of the pipe (1) and symmetrically about the weld centre line.
, Description:A METHOD FOR LOCAL POST WELD HEAT TREATMENT OF INCONEL 617 PIPE
FIELD OF INVENTION
[001] The present invention in general relates to a method for carrying out local Post weld heat treatment (PWHT) in Inconel 617 pipe welds. The present invention in particular relates to heat treatment of Inconel 617 wall of thickness up to 40 mm and those that are fabricated in field conditions.

BACKGROUND OF THE INVENTION
[002] The Inconel 617 (IN 617) material is considered as one of the potential candidate material for advanced ultra-supercritical (AUSC) boiler applications. A typical AUSC boiler is expected to have many piping components made of this material. As per the specification of ASME (American Society of Mechanical Engineers) Boiler and Pressure Vessel (BPVC) code, when an IN 617 pipe of wall thickness exceeding 13 mm is welded then a post weld heat treatment is to be performed for proper tempering of the microstructure and for relaxation of welding induced residual stresses. The soaking temperature for the Post Weld Heat Treatment is defined as a minimum of 950° C in section I of ASME BPVC.
[003] While the PWHT can be conveniently performed in furnaces, when these are fabricated in production shop floors, performing the PWHT in field conditions is highly challenging due to various reasons like conduction losses on account of heat sink effect of the material adjacent to the area subjected to PWHT, convective heat loss to the atmosphere and also radiative heat losses. Moreover, performing a local PWHT at temperatures exceeding 950°C is not a common practice for the currently used boiler grade of materials.
[004] In the present invention, an effective technique of local PWHT of IN 617 pipe welds is provided that uses heating pads, with least possible power input and with least possible through thickness gradient across the pipe wall thickness.

PRIOR ART SEARCH
[005] US patent: US6676777 B2 describes post weld heat treatment process, wherein a welded joint made of carbon steel and low alloy steel is held within austenite single-phase temperature range for a given time and subsequently the joint is cooled by air-cooling or by slow cooling at a cooling rate lower than that of the air-cooling. Whereas the present invention describes a technique of local PWHT of IN 617 pipe welds using resistance heating pads and by using least possible power input for the PWHT operation.
[006] China Patent: CN102864294B discloses a method and apparatus for PWHT of thin-walled tubular parts, using a high-frequency induction heating apparatus as a heat source, using a predetermined high-frequency induction coil inner diameter, with fast heating times of the order 10 – 60 seconds. Whereas, is concerning the local PWHT of IN 617 pipe welds of wall thickness less than 40 mm and using resistance heating pads and employing minimum possible power input.
[007] China patent CN103276188A relates to a heat treatment device, in particular to a PWHT device for mounting a utility boiler water wall. The PWHT device comprises an insulation cotton fixing plate provided with insulation cotton, a heating component arranged on the lower end surface of the insulation cotton fixing plate, a heating plate fixing plate used for fixing the heating component and arranged on the upper end surface of the insulation cotton fixing plate, and an absorption component used for adjusting the absorption strength of the device and arranged on the heating plate fixing plate. However, the present invention is different in the sense that this discloses a special technique of carrying out local PWHT in In 617 pipe welds of thickness less than 40 mm, by employing flexible ceramic resistance heating pads and by using the least possible power input. This does not involve the use of fixing plates containing insulation material.
[008] China patent CN 102796863A relates to a method of a local post weld heat treatment of a steel casting, and particularly relates to a method used for a local post weld heat treatment of a large-size steel casting after re-welding. The method comprises the following steps of: firstly, carrying out hardness detection on a welding zone after re-welding, wherein the hardness does not exceed a critical upper cutoff hardness and thereafter conducting an induction heat treatment for better quality of cast products. Whereas, the present invention relates to a novel technique of carrying out local PWHT of Inconel 617 pipe welds of thickness lesser than 40 mm using flexible ceramic resistance heating elements and by consuming least possible input electrical power.
[009] European patent EP 0034057 B1 discusses a method for post weld heat treatment) of a welded portion of, for example, a thick base metal, and more particularly to a method for appropriately judging the time point for terminating the PWHT when sufficient of the residual diffusible hydrogen in the welded metal has been dissipated by the after heating. But, the present invention relates to a technique of carrying out local PWHT in IN 617 pipe welds with least possible through thickness temperature gradient during soaking. This is accomplished using flexible ceramic resistance heating elements of special types.

OBJECTIVE
[0010] A main objective of the present invention is to provide a technique for carrying out local post weld heat treatment (PWHT) of Inconel 617 pipe welds using flexible ceramic resistance heating elements.
[0011] Another objective of the present invention is to provide a technique for carrying out PWHT in field conditions without loss of much heat to adjoining areas.
[0012] Yet another objective of the present invention is to provide for a technique for carrying out local PWHT of Inconel at a temperature of 950oC.
[0013] Still another objective of the present invention is to provide for a technique for carrying out local PWHT of Inconel 617 that consumes minimum electrical power input.
[0014] Another objective of the present invention is to provide for a technique for carrying out local PWHT of Inconel 617 where the pipe wall thickness do not exceed 40 mm.

SUMMARY OF THE INVENTION:
[0015] In the preset invention a method for post weld heat treatment of Inconel 617 pipe is proposed that can be safely carried out in field condition without much loss of heat to the atmosphere. The method in the present invention comprises of calculation of Heat Band of a pipe that is welded and over which heat treatment is to be carried out. The next steps involve calculating number of heating elements that are required to be wound over the calculated Heat Band of the pipe. After obtaining the number of heating elements, they are wound over the pipe and power is supplied to the heating elements from a controlled power source for heating up the Heat Band of the pipe to a threshold temperature.
[0016] In an aspect, the Heat Band is calculated by equation (1):
HB = (t + W + t) + [4 * (R1*t)1/2]
Where, ‘t’ – Thickness of weld, ‘W’ – Width of widest weld segment on the outer surface of the pipe, “R1’ – Outer Radius of the pipe.
[0017] In an aspect, the heating element are ceramic pad heating element (CP48).
[0018] In an aspect, the calculating of number of heating elements that are required to be wound over Heat Band further comprises of steps of calculating mass of material of the Heat Band (HB) to be heated, using equation (2):
(M) = ? * (3.1416) * HB * (R12 – R02) – Equation (2)
Where,
‘?’ – Density of the material Inconel 617
‘R0’– Inner radius of the pipe
Followed by step to calculate amount of energy that is required for heating the calculated mass of the material using equation (3):
(E) = M * Cp * (980 – AT) – Equation (3)
Where,
‘T’= (980-AT)/(220/t) – Equation (4)
And then power required to heat the heating element is calculated as per the given expression: P= (1.2*E)/T – Equation (5)
Number of heating elements (N)= P/2.9 – Equation (6)
[0019] In an aspect, the method disclosed in the present invention further comprises of sensing of temperature of the Heat Band by a thermocouple that is placed at center of the welded pipe and sending feedback signal having information about temperature of the Heat Band to the power source.
[0020] In an aspect the method disclosed in the present invention further comprises of maintaining temperature of the Heat Band upto a threshold limit using sensed temperature of the Heat Band.
[0021] In an aspect, the method also comprises of step of wrapping of insulation material around the heating elements and adjoining areas to avoid loss of convective heat.
[0022] In an aspect, ‘N’ number of heating pads are wound helically over the outer surface of welded zone of the pipe and symmetrically about the weld centre line.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
[0023] It is to be noted, however, that the appended drawings illustrate only typical embodiments of the present subject matter and are therefore not to be considered for limiting of its scope, for the invention may admit to other equally effective embodiments. The detailed description is described with reference to the accompanying figures. In the figures, the left-most digit(s) of a reference number identifies the figure in which the reference number first appears. The same numbers are used throughout the figures to reference like features and components. Some embodiments of system or methods in accordance with embodiments of the present subject matter are now described, by way of example, and with reference to the accompanying figures, in which:
[0024] Fig. 1 schematically illustrates CP48 snake type flexible ceramic resistance heating pad.
[0025] Fig. 2 is a schematic arrangement of post weld heat treatment setup in pipe weld joint with all constituents.
[0026] Fig. 3 shows steps involved in the method of performing local PWHT in Inconel pipe.

DETAILED DESCRIPTION OF THE PRESENT INVENTION
[0027] These and other aspects of the embodiments herein will be better appreciated and understood when considered in conjunction with the following description and the accompanying drawings. It should be understood, however, that the following descriptions, while indicating preferred embodiments and numerous specific details thereof, are given by way of illustration and not of limitation. Many changes and modifications may be made within the scope of the embodiments herein without departing from the spirit thereof, and the embodiments herein include all such modifications.
[0028] The embodiments herein and the various features and advantageous details thereof are explained more fully with reference to the non-limiting embodiments and detailed in the following description. Descriptions of well-known components and processing techniques are omitted to not unnecessarily obscure the embodiments herein. The examples used herein are intended merely to facilitate an understanding of ways in which the embodiments herein may be practiced and to further enable those of skill in the art to practice the embodiments herein. Accordingly, the examples should not be construed as limiting the scope of the embodiments herein.
[0029] It should be noted that the description and figures merely illustrate the principles of the present subject matter. It should be appreciated by those skilled in the art that conception and specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present subject matter. It should also be appreciated by those skilled in the art that by devising various arrangements that, although not explicitly described or shown herein, embody the principles of the present subject matter and are included within its spirit and scope. Furthermore, all examples recited herein are principally intended expressly to be for pedagogical purposes to aid the reader in understanding the principles of the present subject matter and the concepts contributed by the inventor(s) to furthering the art, and are to be construed as being without limitation to such specifically recited examples and conditions. The novel features which are believed to be characteristic of the present subject matter, both as to its organization and method of operation, together with further objects and advantages will be better understood from the following description when considered in connection with the accompanying figures.
[0030] These and other advantages of the present subject matter would be described in greater detail with reference to the following figures. It should be noted that the description merely illustrates the principles of the present subject matter. It will thus be appreciated that those skilled in the art will be able to devise various arrangements that, although not explicitly described herein, embody the principles of the present subject matter and are included within its scope.
[0031] The present subject matter herein relates to a method for carrying out post weld heat treatment (PWHT) of welds made in Inconel 617 pipes of wall thickness upto 40 mm. In reference to Fig.1, the heating elements used for the method to carry out post weld treatment, are flexible ceramic pad with the CP48 configuration. Fig. 1 describes a general schematic sketch of the CP48 type of resistance heating element 3. The heating element 3, CP48 is approximately of length 1200 mm and of width 45 mm. CP48 are typically rated as 65 V, 2.9 kW.
[0032] Fig. 2 illustrates schematic arrangement of the PWHT setup. In reference to Fig. 2, the heating elements 3 are wound on the outer surface of the pipe 1, on the welded zone and these heating elements 3 are heated up to increase the temperature of weld as per the desired heating ramp rate. The heating ramp rate is defined as the rate at which temperature rises in the pipe during the heating phase of the PWHT process and is expressed as: Heating ramp rate = 220 / (Pipe weld thickness in inches) °C/hour
[0033] Referring to Fig. 2, the terminals of the heating elements 3 are connected through power cables 5 to a power source 6. A thermocouple 2 is placed in the center of the weld (outer surface of pipe 1) and is connected to the power source 6 through compensation cables 7. The thermocouple 2 is designated as “control thermocouple”, since the temperature sensed by this thermocouple is used as a feedback signal by the power source 6 to heat up the weld area as per the desired heating ramp rate. Further, instead of using a very lengthy thermocouple to cover long distances, the compensation cables, makes the job easier in the sense that shorter thermocouples can be used and the rest of the length, the compensation cables are laid for rest of the length. During the heat treatment, in order to get a record of time versus temperature of weld zone, a temperature recorder 4 is used.
[0034] Fig. 3 illustrates the steps that are carried out in the method of PWHT of pipe 1. The method begins with the step of calculating 101 Heat Band that is the width across which the heating elements 3 are wound over the outer surface of the pipe 1. The Heat Band is calculated using the below given formula:
Heat Band (HB) = (t + W + t) + [4 * (R1*t)1/2] – Equation (1)
Where,
‘t’ – Thickness of weld
‘W’ – Width of widest weld segment on the outer surface of the pipe
‘R1’ – Outer Radius of the pipe
The heated band is identified on the outer surface of pipe, in such a way that the center of the weld coincides with the center of the heated band.
[0035] Since the heating elements are available only on the outer surface of the pipe 1, it is required to heat the outer surface of the pipe to a temperature more than 950° C, in order to ensure that the temperature in corresponding spot in the inner diameter side of the pipe reaches a temperature greater than or equal to 950° C. Therefore, the outer surface of the pipe 1 is required to be heated until a minimum of 980° C. This result is obtained after various experimental and FEM trials.
[0036] The next step in reference to Fig. 3 involves calculating 102 number of heating elements required to be wound over Heat band of the pipe 1. This step further involves calculating 103 of mass of material available in the heated band Mass of material in the heated band using below given equation:
(M) = ? * (3.1416) * HB * (R12 – R02) – Equation (2)
Where,
‘?’ – Density of the material Inconel 617
‘R0’– Inner radius of the pipe
[0037] The calculation 102 also involves step to calculate, energy that is required to heat this mass of material and is determined by:
(E) = M * Cp * (980 – AT) – Equation (3)
And time (T) during which the heat is required to be given to be weld material is given by
T = (980 – AT) / (220/t) – Equation (4)
Where,
Cp – Specific heat capacity of Inconel 617 material at ambient temperature
AT – Ambient temperature
[0038] And finally, the power required for heat treatment is found out using the below given expression:
P = (1.2 * E) / T– Equation (5)
A factor of 1.2 is introduced in the above equation to account for the heat losses due to conduction, convection and radiation. For all the above calculations, the values are to be considered in the respective SI units. Then the number of heating elements (3) are calculated 102 using the below given formula:
Number (N) of heating elements 3 to be wound in the heated band, N = P/2.9 – Equation (6)
If N is less than 1, that means one heating element 3 CP48 pad is sufficient for heating the mass of material in the Heat Band (HB). If N is greater than 1, then N number of pads are required for heating the mass of material in the heated band (HB). The use of CP48 as heating elements 3 is highly advantageous in reducing the power consumption because, this covers the entire circumference in few turns whereas the other type of pads covers only a sector of the pipe and will not cover the entire circumference of the pipe. Thus, the usage of CP48 heating elements ensures that the power consumed in the heat treatment operation is as minimum as possible.
[0039] The ‘N’ number of heating pads, so deduced, are wound helically over the outer surface of the pipe in the Heat Band width and symmetrically about the weld center line.
[0040] The winding 106 of the obtained number of the heating elements 3 on the Heat Band of the pipe 1 is followed by wrapping 107 of insulation material around the heating elements 3 and adjoining areas to avoid loss of convective heat to atmosphere.
[0041] After completion of above mentioned steps, power is supplied 108 to the heat elements 3 from a controlled power source to heat up the Heat Band of the pipe 1 upto a threshold temperature. The power source 6 is controlled by a Programmable Logic Controller (PLC) or an Electronic Control Unit and therefore is called as controlled power source.
[0042] Further, the temperature of the heat band is sensed 109 by a thermocouple 2 placed at center of the welded pipe and sending feedback signal having information about temperature of the heat band to the power source. The thermocouple senses the temperature and sends feedback signal to the power source 6 which in turn maintains the temperature by controlling supply of power.
[0043] The next and final step in the present invention is to maintain 110 temperature of the heat band upto a threshold limit by controlled power source 6. The power source 6 uses the feedback signal sent by the thermocouple 2 to maintain the temperature by controlling supply of power to the heating elements 1.
[0044] There are four important parameters of PWHT which needs to be inputted into the program of the power source 6 namely, rate of heating, the soaking temperature, soaking time and rate of cooling. Soaking temperature is defined as the temperature, to which the weldment is taken and held for a defined period of time called as soaking time. In the case of Inconel 617 weldments, the minimum soaking temperature is specified as 950°C.
[0045] The soaking time is dependent on the pipe wall thickness. This is the time required for the heat to diffuse to the full section thickness so as to maintain an equilibrium state of temperature distribution both at the face side (outer surface) and root side (inner surface) of the pipe weld. The soaking time is fixed as below
Soaking time = 1 hour for the first 1 inch of weld thickness
Soaking time = 1 hour for the second 1 inch of weld thickness
Soaking time = 2 Hrs + one spell of 15 mins for every additional inch
[0046] The rate of cooling is defined as: Rate of Cooling = 278 / (Pipe weld thickness in inches) °C/hour. This is the maximum rate of cooling that is allowable in the weldment, after the soaking time is completed, in the PWHT process. The power source 6 has the options of programming all these important parameters. The PWHT cycle is then performed as per the program inputted. On energizing the power source, the weldment will be heated in the heating rate specified. The feedback on the temperature levels reached in the weld is sensed through the thermocouple 2. Thus, with the feedback control in place, the weldment is heated as desired. After completion of heating of the weldment unto 980°C, it is soaked for the period specified as soaking time. After completion of desired spell of soaking time, the weldment is allowed to cool at a rate not exceeding the level specified in the formula mentioned in the previous para. After full cooling of the weld to temperatures below 50° C, the insulation cover is opened out, followed by unwinding of the heating pads. This completes the PWHT process.

[0047] Reference numerals:
1- Pipe
2- Thermocouple
3- Heating element
4- Temperature recorder
5- Power cables
6- Power source
7- Compensation cables