What Is the Best Method for Preheating 4130? In the oil and gas industry, AISI 4130 steel is a widely used material. This material is quenched and tempered for strength and other specific properties. Once the material has been welded, the properties of the heat-affected zone are adversely affected. In order to lessen the effects of welding on 4130, preheating is an essential requirement of the welding procedure. While the use of direct flame is the most prevalent, other commonly used methods include induction and resistance heating, with resistance being the next most commonly employed technique. The purpose of this study is to compare induction, resistance, and direct flame preheating methods on multiple levels. This comparison is based on actual test data derived from preheating the same part with each method. No sales nomenclature or assumed data are used. The final result determined the most effective and efficient preheating method. Methodology A single valve body was chosen for the study because of its mass and its similar configuration to valves typically used in the oil and gas industry. The valve was preheated to an industry minimum of 500°F using typical industry practices for all three methods. Throughout each test, the temperatures on the inside and outside of the valve were monitored and recorded on a data recorder. The thermocouples 52 JANUARY 2014 used with the data recorder remained in the same place for all three tests. Elapsed time was recorded in relation to power used and temperature readings. For each trial, once a temperature of 500°F was attained, the temperature was maintained for one hour and the energy used was recorded. Then, the temperature drop was recorded for one hour with no additional heat input. A Fluke® power meter was installed onto the primary input line just after the fuses at the wall disconnect to measure and record the active energy (in kilowatt-hours) used by the induction and resistance power sources. For the direct flame tests, propane fuel gas was used. The amount of propane consumed was determined using a scale to measure the before and after weight of the propane cylinder. Test Procedures Induction The induction heater uses watercooled cables to conduct high-frequency electric current to electromagnetically induce eddy currents within the material. The electromagnetic currents in the material cause the molecules to excite which generates the heat. As such, the heat is generated within the material compared to the other two methods where the heating sources are applied to the external surface and the heat must then be conducted through the part. This results in J. WALKER, D. HEBBLE, and R. HOLDREN are with Arc Specialties Engineering & Consulting, Houston, Tex. more uniform heating through the part thickness and less radiated heat from the preheated component. First, all valve surfaces were covered by wrapping the valve with an insulating ceramic fiber blanket. Next, an induction heating cable was wrapped around the valve over the blanket — Fig. 1. The cable was not in contact with the block at any point. Since the cables are water cooled they remain approximately at room temperature when properly insulated from the part. The induction heating machine uses thermocouples to monitor the temperature and control the machine’s output. Two control thermocouples were placed on the valve, one on the inside and one on the outside, each within 1⁄4 in. (6 mm) of the thermocouples used with the data recorder. The induction heater controller was programmed to preheat the part to 500°F as quickly as possible, and then maintain that temperature for one hour. The data recorder was turned on, the power meter was set to record, and the induction machine was set to preheat. Both the data recorder and the power meter record time along with the other measurements. Once both thermocouples reached 500°F, the machine was set to maintain for one hour and the time on the data recorder and power meter were noted. After one hour, the machine was turned off and the temperature was recorded for another hour after making note of the time on the data recorder. Throughout the test, the A study compares three preheat methods based on time required, efficiency, safety, and cost BY J. WALKER, D. HEBBLE, AND R. HOLDREN
Welding Journal | January 2014
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