Showing posts with label preheat. Show all posts
Showing posts with label preheat. Show all posts

Wednesday, December 5, 2018

Weld Preheating Low Alloy Steels

Weld Preheating
Low alloy steels are defined as consisting of less than 10.5% Ni, Cr, Mo, and other alloy elements. In general, low alloy steels are required to be preheated to some temperature (TPH), prior to welding. It has been suggested that TPH for any given steel should be about 50 F above the martensite start temperature (MS) for the particular steel being welded. Most low alloy steels, however, have fairly high MS temperatures, making welding at or above them somewhat uncomfortable to the welder, thereby potentially compromising weld quality. For such steels, therefore, manufacturers often opt for TPH temperatures below MS. A case in point is AISI 4130 with an MS of 700 F;  For this steel, federal, military, industry and company specifications typically list TPH temperatures in the 200-600 F range, all below MS.

Why Preheat?

Preheating drives moisture and other contaminants off the joint; moisture, lubricants and other contaminants are sources of hydrogen. More importantly, preheating serves to reduce the rate at which the metal cools down from the welding temperature to TPH. This is so whether preheating is above or below MS. Cooling rate reductions will lead to a general reduction in residual stress magnitudes, and also allow more time for hydrogen removal.

Most low alloy steels that may be susceptible to hydrogen-induced cracking transform from austenite during cooling through the 800-500 C (1470-930 F) temperature range. The length of time a steel spends in this range during cooling, will establish its microstructure and, hence, its susceptibility to cold cracking. To maximize cracking resistance, a microstructure that is free of untempered martensite is desired; that is, the austenite would have transformed to ferrite + carbide and no austenite will be available to transform to martensite upon reaching MS.

For more information about preheating low alloy steels, contact Hotfoil-EHS at 609-588-0900 or by visiting their web site at https://hotfoil-ehs.com.

Saturday, November 10, 2018

Mobile Generator and Power Console Trailers by Hotfoil-EHS


Hofoil-EHS manufactures mobile generator trailers for welding heat treating power and temperature control. Custom designed from large to small, Hotfoil-EHS will build to your specification.

609-588-0900

Friday, January 15, 2016

Ceramic Heater Pads for Weld Preheating and Post Weld Heat Treating

welding ceramic pad heater
Welding ceramic pad heater
(courtesy of HotfoilEHS)
The goal of preheating and post weld (PWHT) heating is to maintain temperature between weld passes and to uniformly control changes in target temperatures. By properly achieving, maintaining, and then cooling weld temperatures, you lower weld stress, distortion, reduce shrinkage stress and allow unwanted hydrogen to escape.

It is very important to understand the material characteristics of the metals being welded, and know minimum and maximum preheat temperatures, particularly in tempered steels.

For most preheating, post heat treating, and interpass heating, precise temperature control isn't required. Its more important to maintain a minimum temperature, and stay within an acceptable range during the actual welding process, as well as during cool down.

Exceptions are with tempered steels. Tempered steels have already been heat treated at the steel mill and applying too much preheat can alter that tempering. In these cases, proper set point, temperature limiting, and temperature ramp rate of the welded part is critical.

Use of Ceramic Heater Pads

Resistance heating pads are constructed of ceramic beads strung on nichrome wire. These resistance heating elements accurately raise the workpiece temperature to the proper temperature before, during, and after welding, complying with recommended preheat, interpass, and PWHT practices. These semi-flexible ceramic heater pads, with their interlocking beads and high temperature wire, allow for a fit conforming to the shape of the workpiece, and are capable of temperatures up to 1,850 degrees F. Ceramic mat (pad) heaters have an additional benefit in that they don’t have to be moved during welding.

welding temperature controls
Recorders and controls used for
welding preheat and post heat.
Electronic temperature controllers use several thermocouples spot welded to the workpiece to monitor and regulate the actual part temperature throughout the operation. Many times the electronic controllers have ramping or temperature profiling capabilities, so that heat-up and cool-down can be carefully controlled. Recorders are often used to produce a record of the temperature profiles over time before, during, and after welding. This is important when welding jobs require careful documentation.

An average application is as follows: the heater pad is wrapped around the workpiece, and insulation is applied to the weld joint and the temperature controller is set. Once preheat temperature is achieved, a welder removes the insulation and starts their work. After the weld, the ceramic heaters can be placed over the weld and the controllers can be reset for proper PWHT.