For many years, heat treating critical welds made with alloy steels that have gained wide acceptance in the power industry has been a tedious and exacting procedure. A new technology now makes the process more precise, less expensive and provides superior documentation for such welding projects.
The use of 9 percent chromium-molybdenum-vanadium high-strength alloy steels, such as P91 and P92 grades have become standard within the power generation industry for high-temperature steam and other non-corrosive service. Both of these alloys retain their strength at elevated temperatures and possess good fabrication characteristics.
For example, at temperatures higher than 1,000 F, P91 can withstand sufficiently high levels of stress to be used in coal-fired power plants that operate at the higher pressure and temperatures of main steam piping. The specification of P91 has increased for main-steam lines and heat-recovery generators because they reduce material thickness, purchase cost, result in lighter weight components and produce higher thermal efficiencies. P91 can also be used to replace boiler header sections and pressure parts that occasionally reach temperatures higher than permissible design limits for P92 or other low-alloy chromium-molybdenum-vanadium steels.
Wireless technology replaces cables and labor-intensive labor with computers. Illustration courtesy of SuperheatFGH.
These alloys achieve their strength and hardness because of the austenite-to-martensite transformation that occurs in such alloy steels during rapid cooling from elevated temperatures. If this transformation were not controlled, welds using these alloys would be too hard and brittle. That means that many alloy steels and heavy section work pieces would be at risk of hydrogen cracking during the welding process or later while in service in the presence of hydrogen and/or residual stresses inevitably caused by welding.
Since temperatures of transformation are unavoidable during welding, the cooling rates experienced in the heat-affected zone during welding must be controlled. Industry codes, such as the American Welding Society (AWS) and American Society of Mechanical Engineers (ASME), have adopted heat treatment (HT) requirements tailored to the alloy and section size. For welding advanced alloys, these requirements typically include preheat, holding an interpass temperature during welding, cooling rate control after welding and another heating-cooling cycle to produce final metallurgical transformations and mechanical properties.
“Unfortunately, the traditional heat-treating method has varied little over the past 40 years,” says Gary Lewis, director of business development for SuperheatFGH Services, Inc. “The tried-and-true HT process utilizing thermocouples, ceramic heaters, insulation, and power sources have continued to be used to perform HT metallurgy.”
To create heating/holding/cooling cycles, traditional procedures have used resistance-heat cables connected to power supplies and wrapped through ceramic pieces insulating the cables from the work piece. Actual temperatures are monitored by a controller connected to thermocouple capacitor discharges welded to the work piece. Their dynamic temperature output provides on-off control to the heater cables in order to duplicate the desired temperature-time cycle.
Last year, SuperheatFGH introduced a process that uses wireless data communication technology that might revamp the way these highly complex HT welding processes are conducted. The technology is designed for applications in all segments of the power generation industry and, in general, to support the fabrication of piping and components. Applications include the use of 9Cr-Mo-V steels for boiler tube sections and associated piping, steam headers, and various structural and vessel sections.
“As wireless capabilities have changed how people use computers at home or in the office, the new wireless HT technology replaces cables and costly labor-intensive processing with embedded solid-state computers that produce significant quality and productivity benefits,” says Lewis.
Each function of the new HT process incorporates features that provide quality-focused HT results (Figure). Temperature data flow from Type K thermocouples through a wireless local area network (LAN) to an unlimited number of Super 6Wi heat treating units and to site access management (SAM). Each unit manages up to six heated zones whose thermocouples continuously transmit actual temperatures sampled four times per second. Binary-coded data are sent over the LAN to SAM every 60 seconds.
At the heart of the new HT system is its SuperManager software. Comparing actual and pre-loaded data, it recognizes deviations, rectifies them and notifies designated personnel, thus averting HT failure and the need for costly rework. Control and automatic adjustment capabilities include unit shutdown in the event of over-temperature, an equipment temperature alarm with shutdown protection, circuit-breaker protection and manual emergency shutdown buttons on all units. Should the need arise, monitoring personnel can communicate with the customer’s on-site personnel.
The embedded computer within each unit is able to run the entire job, helping to ensure process continuity and integrity to meet customer specs even in the event of a communications network disruption. The computer also manages alarm functions, which signal conditions such as broken T/C wires, disconnected cables and temperature errors.
Up to 100 units may be used on the typical large power plant site. Within each, temperature data are checked for dynamic accuracy four times a second and the application of power is controlled accordingly. Actual data are logged for six months, and in the event of faults, the unit displays “9999” and cuts off power.
During normal operations, the actual temperature is displayed and power is applied as necessary to maintain the correct temperature. Thereafter, these data are sent over a secure wireless wide area network (WAN /Internet) to the company’s quality management center where temperatures are again verified and archived.
The system can provide periodic text messages with real-time temperature data, delivered to computer, personal data assistant or digital cell phone. This provides the end-user with information on the actual process status for a particular job, any time from anywhere and for any desired time interval.
Welding such critical steel alloys, however, mandates stringent control of pre-heat, interpass and post-weld temperatures. The system’s Smart Light indicator assists the welder and assures quality and adherence to ASME and any relevant codes governing weld temperature. Magnetically attached to a convenient location for the welder’s viewing, a high-visibility LED indicator gives the welder real-time workpiece temperature and guides actions by the welder based upon simple observation of the light. A slow blinking light means the heating process is on and either ramping up or cooling down, telling the welder to stand-by. A solid, unchanging light means the temperature is within specification and it’s OK to weld. And rapid flashing means an alarm condition exists.
All data are verified, stored on site for two weeks and permanently archived off-site. A warning system notifies on-site supervisors with job setup results. For example, if a job requires achieving an 1,150 F temperature, but only 600 F is reached after applying 80 percent power, the computers send a notice that something is wrong and that the desired temperature will not be reached. Thus, power is cut, and a corrective action may be permitted to salvage the integrity of the job.
The quality support staff also manages all necessary quality assurance (QA) documents. Calibration certificates are provided based upon actual on-site calibration verifications. Additional documents include pre-work order forms, customer support documents, secured data reports and charts, Brinnel harness results and work acceptance forms – all delivered as hard-copies or email files.
Using traditional HT, large numbers of on-site technicians conduct many rudimentary tasks, including passively monitoring paper charts during long HT cycles. Such a large use of manpower and associated paperwork adds to project cost. Wireless technology can yield savings by eliminating direct and indirect costs for cable/wire installation and inevitable facility/HT cycle disruptions. It can reduce the numbers of on-site workers, meaning fewer background checks, risks and associated liability.