New power plants are in high demand and several hundred can be expected to be built over the next decade. Figure 1: Postweld heat treatment requires controlling temperature in four phases to relieve the stress caused by welding. As a result of nationwide power emergencies and an abundance of natural gas, 300 to 400 new power plants are projected to be built by the end of this decade.
Whether building a new facility or upgrading an old one, utility companies and designers want to reduce fabrication and materials costs. Welding P91 generally requires preheating the joint, maintaining interpass temperatures, hydrogen bakes, and postweld heat treatment (PWHT). Preheating also reduces the thermal gradient between the base material and weld puddle, and it improves weldability by reducing the heat necessary to make the weld while reducing hot-cracking tendencies. Welding thicker materials and high-alloy steels usually requires maintaining minimum preheat and maximum interpass temperatures. PWHT requires controlling temperature in four phases to relieve the stress caused by welding.
According to Newell, "Prior to stress relieving, it is not unusual to see an as-welded P91 weldment with a Rockwell C hardness in the mid 40s to low 50s and a Charpy V-notch toughness of just 3 foot-pounds.
Newell says he cannot emphasize enough that P91 provides no forgiveness when fabricators and installers fail to stress-relieve the material properly.
Also, like most heavy-wall, large-diameter piping, a P91 pipe's inside diameter (ID) and outside diameter (OD) can exhibit a substantial temperature gradient (50 to 100 degrees) if the procedure and technique are not carefully evaluated and qualified, so installers need to bring ID and OD temperatures as close together as possible. Today's more advanced induction heating systems use compact, lightweight, and energy-efficient inverter technology. According to Newell, technology advancements have simplified the system so that an operator can be taught how to use it in minutes. He says that with all methods of postweld heat treatment, experience with the diameter of the pipe, thickness of the material, orientation of the weldment, and other influences such as chimney effects that might influence heating and cooling must be thoroughly evaluated and taken into consideration. As for preheating, Newell says, "Induction heating preheats the material in a short amount of time and can easily control interpass temperature during welding.
In the fall of 2001, a 25-kilowatt induction heating system was used on a project that involved installing a new reheat outlet header (high-pressure steam flows through the header into the turbine).
According to ASME Section 1, those temperatures were preheat and interpass temperatures of 450 degrees and a PWHT temperature of 1,300 degrees for 105 minutes.
Time also can be lost if the heating system loses power in the middle of a stress-relieve cycle. A 25-kW induction system (power source, controller, cooler) draws 45 amps on 460-V primary. The FABRICATOR® is North America's leading magazine for the metal forming and fabricating industry. Electrical transformer is a static electrical machine which transforms electrical power from one circuit to another circuit, without changing the frequency.
Transformers can be classified on different basis, like types of construction, types of cooling etc.
In core type transformer, windings are cylindrical former wound, mounted on  the core limbs as shown in the figure above. Step down transformer: Voltage decreases (with subsequent increase in current) at secondary.
Distribution transformer: Used in distribution network, comparatively lower rating than that of power transformers.

Chromium-molybdenum steel pipe (P91) is a major construction material in these plants and there are particular methods for welding it properly to insure it will hold up. In addition, utility companies want systems that can withstand higher temperatures and operating pressures to increase operating efficiency and output.
Preheating, typically to 400 to 500 degrees F, drives off moisture and thereby reduces hydrogen. If a material cools below the minimum temperature between passes, restraint issues may arise or hydrogen may contaminate the weld. The following example describes the four phases with P91 and follows ASME B31.1 (see Figure 1). The weldment usually may be brought to this temperature, generally 600 degrees, without any time constraints. Once the control temperature is reached, the code requires a controlled temperature rise of no more than 600 degrees per hour divided by one-half the maximum thickness, but in no case more than 600 degrees per hour.
After the soak time, the material must be cooled at a controlled rate to prevent stressing the part, which could lead to cracking. Without ductility at ambient temperature, power piping and other chromium-molybdenum components may not withstand hydrostatic testing and other aspects of fabrication, transportation, start-up, and shutdown phases. Modern induction heating power sources weigh from 165 to 500 lbs., and some can fit on a small, wheeled cart along with a temperature controller and recorder and water cooler (see Figure 2). The beauty of induction technology is that it maintains temperature without artificially injecting more heat than needed.
The rapidly changing magnetic field creates eddy currents in the part, causing heat to be generated. For quality assurance, the induction heating system's thermocouple temperature data was recorded on a floppy disk and could be recalled for verification purposes.
Because of their low power requirements, inverter-based induction heating systems are unlikely to trip circuit breakers, which would demand starting the stress-relieving process all over again, as code requires.
The magazine delivers the news, technical articles, and case histories that enable fabricators to do their jobs more efficiently. Transformer can increase or decrease the voltage with corresponding decrease or increase in current. The steel used is having high silicon content and sometimes heat treated, to provide high permeability and low hysteresis loss. A shell type transformer may have simple rectangular form (as shown in above fig), or  it may have a distributed form.
The NDT (Non-Destructive Testing) Division of Eddysun has been specially engaged in the development, research, production and application service of NDT equipment.
Preheat and postweld heat treatment are mandatory and absolutely critical to the service life of the alloy.
If the material becomes too hot, which can happen during multiple-pass welding, some steels can lose their corrosion resistance. However, always refer to the appropriate code for procedural details and never work from memory. Typically, the control-cooling rate is the same as the control-temperature rise — cool from 1,400 to 600 degrees at a rate of no more than 400 degrees per hour. Induction heating systems of the 1970s used power sources the size of a small car that required enormous amounts of primary power. Basically a transformer consists of two inductive coils; primary winding and secondary winding.

A transformer may also consist of a container for winding and core assembly (called as tank), suitable bushings to take our the terminals, oil conservator to provide oil in the transformer tank for cooling purposes etc. On the basis of several decades of experience in the technical research and application service of NDT equipment, by unceasingly importing and studying NDT advanced technologies in the world and creating new detecting concepts, We have formed a series of representative techniques with Chinese characteristics, like frequency spectral analysis, pre-multi-frequencies current, multi-frequencies eddy current, remote field eddy current, metal magnetic memory, ultrasonic, quick sound pulse leakage detection, acoustic impedance, phase array and NDT intelligent network.
Also, a wall that is 50 percent thinner takes less time to weld and uses less filler metal. The old systems also had copper tubes to carry the induction energy and for cooling water flow. Our company has produced advanced metal magnetic memory diagnostic instruments, sound pulse leakage detecting equipment, 128 channel eddy current testers, array eddy current transducers, multi-channel real time ultrasonic systems, high resolution and high penetration ultrasonic systems, acoustic oscillation testers, as well as high speed and real time eddy current detection and intelligent network detecting analysis systems approaching to the international level. Application--This intelligent tester is designed for real-time effectively defect detection in metal materials, thickness measurement and sorting materials by metallurgical differences, and weld inspection.-- It can be used to test conductive materials and measure the coating thickness.
When, primary winding is connected to a source of alternating voltage, alternating magnetic flux is produced around the winding. To avoid high reluctance at joints, laminations are stacked by alternating the sides of joint.
Our company has much scientific research achievements, more than 30 patents and patented utility models. EDDYSUN own the advanced mixing algorithms, providing the cleanest, most effective signal mixes.-- The audio, visual and logic output alarms can determined flaw conditions.
That is, if joints of first sheet assembly are at front face, the joints of following assemble are kept at back face. Eddysun strictly organizes NDT instrument production and quality management according to ISO9001, and is a qualified equipment supplier for Daya Bay Nuclear Power Station, Air Force, Sea Force, Aviation Force of Ground Force and others. Real-time C-scan display helps you to differentiate between multiple layers and determine flaw orientation.
Our company is also the selected NDT supplier for National Supervision Bureau of China for Quality and Technique, Nuclear Testing Base, and Carrier Vehicles Research Institute of China.
This emf is called 'mutually induced emf', and the frequency of mutually induced emf is same as that of supplied emf. At the same time, our company is an important sector recommended for participating in National Patent Achievement Exhibition and is appointed to applying and popularizing high and new technical products of Scientific and Technical Committee of National Defense Industry. If the secondary winding is closed circuit, then mutually induced current flows through it, and hence the electrical energy is transferred from one circuit (primary) to another circuit (secondary). Products of our company serve effectively for domestic and abroad enterprises, and are sold to the United State of America, India, Thailand, Iran, Korea, Malaysia, Indonesia, Singapore, Argentine, Australia, Hong Kong and Taiwan area. Obvious economic and social benefits have been achieved.Eddysun is committed by NDT Society of China to being in charge of writing and editing the Eddy Current Education Information. More than 300 technical papers written by our engineers have been published in professional journals. Our company participates in or is responsible for the revision and establishment of many national standards. Can match with both absolute and differential probes, such as feed-through probes, pencil probes etc.13. Display both the number and the eddy current signal of the testing tube on the screen at the same time19.

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