An Introduction to Thermocouples

Schematic of Type K Thermocouple
(courtesy of Wikipedia)

A thermocouple is a temperature sensor that produces a micro-voltage from a phenomena called the Seebeck Effect. In simple terms, when the junction of two different (dissimilar) metals varies in temperature from a second junction (called the reference junction), a voltage is produced. When the reference junction temperature is known and maintained, the voltage produced by the sensing junction can be measured and directly applied to the change in the sensing junctions' temperature.

Thermocouples are widely used for industrial and commercial temperate control because they are inexpensive, fairly accurate, have a fairly linear temperature-to-signal output curve, come in many “types” (different metal alloys) for many different temperature ranges, and are easily interchangeable. They require no external power to work and can be used in continuous temperature measurement applications from -185 Deg. Celsius (Type T) up to 1700 Deg. Celsius (Type B).

Thermocouple attachment unit

For pre-weld heat treatment, Type K thermocouples are generally used. Thermocouples can be directly attached to the workpiece by spot welding the junction of Type K thermocouple wire. The attachment is done with the help of a TAU, or thermocouple attachment devices. Thermocouples are attached so that there is firm contact between the sensing tip and the assembly being heat treated.

Welding Preheat Basics

Why preheat welds?

Weld preheating is the process of heating the base metal (parts to be welded) to a specific temperature prior to welding. The specific temperature to which the part needs to be heated (before welding) is referred to as the “preheat temperature”.

The area requiring preheat may be the whole (entire) part, or just the area immediately surrounding the weld.

Preheating may continue during the actual welding process, but many times the energy generated from welding will be sufficient to maintain the desired temperature.  The temperature of the weld between the first pass and the last pass is referred to as “interpass temperature”. As long as it can be assured that interpass temperature will not fall below the preheat temperature, continued preheating is usually not required.

There are several key reasons why it's important to preheat before welding. First, a preheated part cools more slowly, which slows the overall cooling rate of the welded part. This improves the metallurgical (crystalline) structure and makes it less prone to cracking. Additionally, hydrogen that may be present immediately after a weld is also released more efficiently, which further reduces the possibility cracking. Preheating also mitigates stress from the shrinkage at the weld joint and nearby metal. Finally, pre-heating reduces the possibility of fracture during fabrication due to brittleness.

Electric welding preheaters, known as "ceramic mat heaters", are rugged and flexible heating elements designed so that they conform uniformly around the weld and surrounding area.  Ceramic mat heaters are normally controlled by a power console that uses thermocouples and electronic controllers to regulate, monitor, and many times record, the preheat temperature profile.

Welding code is the first determinant to whether pre-heating is needed. Welding code carefully specifies the minimum preheat temperature, the soak time, and the welding process. Many criteria are considered by welding codes, all gathered from years of rigorously tested data. This data is accumulated from many sources, including metallurgical science, chemical properties of materials, and radiographic analysis.

Determining whether or not preheating is required should not be taken lightly, as it is critical to the quality of a weld and therefore critical to the performance of a structure. When in doubt, review of industry code or contacting an industry expert, is imperative.

Automatic Heat Treatment Power Consoles

Typical Power Console
(courtesy of HotfoilEHS)

Automatic heat treatment power consoles are used to control various heat treatment processes (i.e. pre-weld, post-weld) by closely controlling the temperature of the item being welded. The power console accurately controls the ramping rate (up and down), the soak temperature, the set point and the time. Power consoles are available from 2 to 24 zones of control. Zone can be used either in the fully automatic or manual mode.

Ceramic Mat Heater

The power console is used to provide power to electric heating elements called ceramic mat heaters. Ceramic mat heaters are constructed of nichrome wire interwoven into ceramic beads which provides electrical insulation and protection. These heaters are quite rugged and conform to curved and irregular shapes.

Thermocouples are used to sense the target temperature and send their signal back to some type of electronic temperature controller, recorder, or combination thereof. The sophistication of the control system can range from simple manual control to fully automatic control with large graphic displays. Recorders are frequently used to document the pre-heat, soak, and post-heat process. Welding integrity depends on precise and accurate control.

Recorder used on
welding power console.

Heat treatment power consoles are built on sturdy chassis of steel and depending on ambient conditions, stainless steel. Construction includes wheels and handles for easy relocation and many electrical components for safety and convenience (such as amp meters, indicator lights, cut-off switches, fuses, and alarms).

For more information, contact:

Hotfoil-EHS, Inc.
2960 East State Street Ext.
Hamilton, NJ 08619
Phone # 609.588.0900
Fax # 609.588.8333
www.hotfoilehs.com
Email: dap@hotfoilehs.com

Welding Application Note: Demand Pulse Technology Saves Three Weeks Production Time

Demand Pulse technology
welder (courtesy of Aftek EHS)

Demand Pulse is a unique, patented welding process. Because of the low peak current, open butt pipe welds are easier in all positions.

Short arc and pulsed spray both have peak currents in the 375+ amp range, and are at peak many times longer than the Demand Pulse process.

With Demand Pulse, welding current and volts are similar to those seen when “short-arc” welding, but there are NO short-circuits - the arc never goes out, and cold laps are almost impossible.

Demand Pulse allows for the use of very light torches, and still experience extremely long tip life. A 140 amp hand torch will not overheat running 200 amps on overlay work. Its even been reported that some customers only use one tip per week with Demand Pulse,  as opposed to several per day using conventional welders.

The first two pictures below are of an initial job done by a new customer using Demand Pulse MIG. The large skids pictured have (580) open butt, 316 SS pipe welds. The smaller manifold shown has (24) welds, for a grand total of (604) welds.

Only two qualified welders worked on this demanding job. They tested other types of equipment, all with unsatisfactory results. Then, after testing a Demand Pulse system, a good coupon was produced on just the second try, so the customer purchased a system and production started immediately upon delivery.

Astoundingly, the job was completed three weeks ahead of schedule, saving the fabricator 1600 hours of production. Most importantly, there wasn't a single repair required, and all welds passed 100% X-ray to ASME Code.

Large skids with 580 welds, all open butt 316 pipe.

Small manifold with 24 welds.

After experiencing Demand Pulse, this customer commented he'll never use anything else and that the welder paid for itself on the first job.

Below are close-ups of two of the welds on the large skid. The customer insisted on TIG wash on the cap. The TIG cap had 7 repairs - welds using Demand Pulse needed NO repairs.

TIG welds need 7 repairs.

Demand Pulse required 0 repairs.

For more information, contact:

AFTEK-EHS Welders
2960 East State Street Ext.
Hamilton, NJ 08619
Phone # 609.588.0900
Fax # 609.588.8333

www.hotfoilehs.com
Email: dap@hotfoilehs.com

Check Out North America's Largest Metal Forming, Fabricating, Welding and Finishing Event

Come visit HotfoilEHS at FABTECH

The annual FABTECH exhibition is happening in Chicago this year on November 9th through 12th, 2015.

FABTECH is sponsored by the the Fabricators & Manufacturers Association (FMA), SME, Precision Metalforming Association (PMA), Chemical Coaters Association International (CCAI), and the American Welding Society (AWS).  There will be 1500 companies exhibiting all kinds of forming & fabricating, tube & pipe, metalform, welding, thermal spray, and finishing equipment.

HotfoilEHS is proud to be exhibiting their power consoles, ceramic mat heaters, welders, GRID systems, transformers and power supplies again this year at Booth N27044.

It's a great event where you can see the latest industry products and developments, and find the tools to  improve productivity, increase profits and discover new solutions to all of your metal forming, fabricating, welding and finishing needs.

For more information on FABTECH, visit http://www.fabtechexpo.com.