Portable Hardness Testing for Field Welding

Portable Hardness Testing Kit

Hardness testing is an easy and informative test to learn important information about material properties. For most general applications, hardness can be described as the property a material has to resist indentation. In welding, its important to measure hardness of welds for quality and material compliance.

Various methods and procedures are used to measure hardness. One method is to create a "dent" on a target material (under controlled conditions) and then carefully measure the resulting indentation. The indentation correlates directly to the hardness of the material. In very general terms, the smaller the mark, the harder the material, and the larger the indentation, the softer the material.

The Brinell Hardness Number (BHN) and Brinell testers are very popular. Brinell testing is done by pressing a perfectly spherical ball (10 mm in size) into a target material with a force of over 6600 lbs (3000kg to be exact) for a minimum of 10 seconds, and then measuring the resulting impression. After which, the size of the impression is measured by an optically scaled microscope positioned over the impression.

Laboratory Brinell testers are obviously impractical in the field, but portable, lightweight Brinell testers are available for weld hardness testing in the field. These portable testers are easily transported and simple to use.  No special training is required and in a few minutes anyone can learn to determine a Brinell Hardness Number accurately.

The accuracy of portable Brinell testers are done via NIST traceable test bars. The test bars are calibrated to a uniform hardness of ±3% of their labeled Brinell Hardness Number. Additionally, the microscopes used for measurement are capable of reading within .05 millimeters.

Typical Field Procedure

A test bar with a known BHN, and approximately the hardness of the target material to be tested, is chosen. The test bar is inserted into the Brinell test instrument and the instrument placed upon the specimen.  A steel impression ball, secured in the the head of the instrument, is in contact with both the test bar and the target material. Built-in to the test instrument is an anvil, which is then struck sharply with a hammer. Via the impression ball, the impact of the hammer, regardless of force, is transmitted equally to the test bar and to the target material, leaving marks in both. The diameters of the resulting indentations are directly related to the respective hardnesses of the test bar and the specimen.  The bar is removed from the test instrument and the microscope is positioned over the appropriate indentation.  The impression in the specimen metal is measured in the same manner and the hardness is determined.

For more information on portable Brinell Harness Test kits, contact:

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

Postweld Heat Treatment

Some welding operations require postweld heat treatment

Welding is a process of joining materials, usually metal or thermoplastic, by causing a fusion at the junction of two or more physical parts. The fusion is accomplished through the creation of specific conditions at, and adjacent to, the joint area. Welding is a heat based process, but there are other heat based processes used for joining metals. The welding process differs from lower temperature joining operations, such as soldering or brazing, that do not require melting of the base metal.

There are numerous welding methods, as well as specific procedures that are applicable to different metal alloys and intended use of the finished product. Some of these procedures involve controlled heating and/or cooling of the weld and the closely adjacent area of the parts being joined. This is generally termed heat treatment, and more specifically, preheating and postweld heat treatment.

Heat treatment, in any form, is a time consuming and costly operation. Strength, toughness, corrosion resistance, and residual stress of the joint are all impacted by the proper application of heat treatment. Many application codes and standards require specific protocols for heat treating, which is utilized to achieve a specific resulting condition in the subject material or to restore material characteristics altered by production processes such as welding.

Stress relief is a benefit and goal of postweld heat treatment. During the welding process, base materials near the weldment, deposited weld metal, and the heat affected zones exhibit different levels of metallurgical phase transformation. Various degrees of material hardening occurs, particularly in the heat affected zone adjacent to the weld metal deposit, with high stresses due to melting and solidification. Stress relief through heat treatment is accomplished by controlled heating and cooling of the affected material area to relieve a significant amount of the imposed stresses. This postweld heat treatment can reduce the hardness and increase ductility, lessening the incidence of cracking in weldments. The process generally involves maintaining the target area at a specific temperature (soaking), then executing a controlled cooling schedule.

One respected source, TWI, sums up post weld heat treatment (PWHT).....

The necessity for PWHT depends on the material and the service requirements. Other factors that influence the need for PWHT are the welding parameters and the likely mechanism of failure. In some standards, PWHT is mandatory for certain grades or thicknesses, but where there is an option, cost and potential adverse effects need to be balanced against possible benefits. The energy costs are generally significant due to the high temperatures and long times involved, but costs associated with time delays may be more important. Detrimental effects include distortion, temper embrittlement, over-softening and reheat cracking, which means that control of heating and cooling rates, holding temperature tolerances and the times at temperature are extremely important, and must be carefully controlled in order to realise the full benefit of the process.

The precise level of heat control, as well as the substantial quantity of heat, required to successfully accomplish a PWHT protocol calls for the use of special purpose equipment intended for PWHT. Hotfoil EHS is a recognized manufacturer of industrial application heating equipment and specializes in the design and manufacture of power consoles, ceramic mat heaters, and accessories for pre and post-weld heat treatment. Share your application challenges with them and work together to develop a cost effective solution.

Rely on Experienced Welding and Heating Applications Experts for Improved Outcome and Efficiency

Work with the manufacturer's experts for better outcomes

When it comes to selecting the right equipment for welding and industrial heating jobs, projects and tasks are best completed and accomplished through the proper application of the right resources. There exists an access point to high level technical knowledge and assistance that can be easily tapped and brought to bear on your successful task or project completion. The manufacturer's own internal experts.

 
Manufacturers provide services that may help you save time and cost, while also achieving a better outcome for the entire project. Consider a few elements the technical sale rep brings to your project:

• Product Knowledge: Product managers and sales support personnel will be current on product offerings, proper application, and capabilities. They also have information regarding what products may be obsolete in the near future. This is an information source at a level not generally accessible to the public via the Internet.
• Experience: As a project engineer, you may be treading on fresh ground regarding some aspects of your current assignment. There can be real benefit in connecting to a source with past exposure to your current issue. 
• Access: Through the manufacturer's internal applications people, you may be able to establish a connection to “behind the scenes” information not publicly available. The people at the manufacturer can provide answers to your application questions.

Certainly, any solutions proposed are likely to be based upon the products sold by the manufacturer. That is where considering and evaluating the benefits of any proposed solutions become part of achieving the best project outcome.

Develop a professional, mutually beneficial relationship with the manufacturer's technical sales team, and don't be shy to develop a professional and trusting relationship. Their success is tied to your success and they are eager to help you.

Resistance Heating Mats for Welding Pre and Post Heat Treatment

Ceramic Mat Heater

Resistance heating mats (or pads) are constructed of ceramic beads strung on nichrome or Kanthol wire. These resistance heating elements accurately heat the workpiece to the proper temperature before, during, and after welding, assuring compliance with recommended preheat, interpass, and PWHT practices. Designed as semi-flexible ceramic heater pads, with interlocking beads and high temperature wire, so a proper fit to the workpiece. Capable of temperatures up to 1,850 degrees F., ceramic mat (pad) heaters have an additional benefit in that they can withstand welding temperatures and don’t have to be moved during welding.

Resistance Heating Mats for Welding Pre and Post Heat Treatment from Hotfoil EHS, Inc.

Air Carbon-arc Gouging: A Fast and Efficient Way to Get Rid of Metal

In metal working maintenance and repair, it is sometimes required to repair or replace a weld, or remove excess metal from a worn or defective part. A process called air carbon-arc (also known as air arc) gouging, developed in the 1940’s, has become a widely accepted method for metal working. Compared to grinding, chipping, and cutting, air arc gouging provides a much faster, more efficient, and more cost-effective means to remove unwanted metal.

Air carbon-arc gouging differs significantly from oxy-fuel cutting (OFC) and plasma cutting. Air carbon-arc does not require oxidation to maintain the cut and is able to be used on many kinds of metal. Air carbon-arc gouging cuts and removes metal by an electric arc created a carbon or graphite electrode as it is drawn along the target metal. As the arc melts the target metal, a steady, high velocity air stream blows the molten material out of the way. The arc is supported by a constant current power source. A compressed 60 to 100 psi gas source supplies the air stream. A special air arc torch is required, as it not only holds the electrodes, but has unidirectional air ports built in to direct the air stream.

Air carbon-arc gouging will work on any material that will conduct electricity and which can be melted by the electric arc, such as carbon steel, stainless steel, cast irons, and many copper alloys. Metal removal rate is controlled by increasing the gouger’s amperage, slowing down the movement of the electrode, and by the efficiency of the air stream. Most common uses for air arc gouging are cutting, removal of defective welds, removal of bolts, removal of rivets, making holes, and casting finishing.

If you have any questions about air carbon-arc gougers, contact:

AFTEK-EHS Welders
6121 Airways Blvd.
Chattanooga, TN 37421
Phone # 423.424.0515
Fax # 423.424.0518

FM Approved Silicone Rubber Heaters for Tank and Vessel Heating

Silicone rubber electric heating element for tank and vessel heating.

Silicone rubber electric heating elements are well proven, tried and tested flexible heaters for heating plastic or metallic tanks and vessels. Their low mass offers virtually no thermal inertia and within minutes can achieve elevated temperatures. They are generally easy to apply to the surface of tanks, chutes, or hoppers and can be custom designed in virtually any shape, all that is required is a sketch of the heated area and information about the electrical connections.

Silicone rubber electric heating elements are constructed by vulcanizing a nichrome heating element to the silicone rubber layer to produce a homogenous laminate. Two layers of 0.028” thick silicone rubber compound, sheath cured on one side, are calendared with high strength fiberglass mesh and vulcanized together under heat and pressure. This bonding sandwiches the resistance heating element between the two layers. An electric grounding mesh is located on the assembly and a further layer of silicone rubber is applied and vulcanized, making a homogenous assembly.

The heaters are used primarily for metallic and non-metallic tank and vessel heating. Their flexibility, both physically and design-wise, allow the heaters to be used for a wide variety of applications. Silicone rubber heaters are water-resistant and may be used for outdoor or “wet” applications, as long as the thermal insulation or exterior covering over the heaters is weatherproofed.

Silicone rubber heating systems must be correctly temperature controlled. The controls and all electrical components must be approved for the area as classified. All wiring must follow Federal, State and Local codes and be fully compliant with the NEC as it relates to the project. If in doubt, please contact an industrial heating applications expert for assistance.

For more information about silicone rubber tank and vessel heaters visit this link.

Pre Weld and Postweld (PWHT) Heat Treatment Furnaces

Industrial furnaces are used for pre and post-weld heat treating by welders and fabricators to heat-treat weldments or metallic parts. Industries that require these types of furnaces include pressure vessels and piping, storage tanks, building construction, bridge building, offshore platforms, petrochemical plants, power plants, and oil and gas refineries.

By using furnaces for PWHT,  stress introduced by the welding process is reduced and redistributed through the heating, soaking and cooling the weldment/machined surface. This greatly improves the weld properties. PWHT is most times mandatory in order to comply with welding codes and specifications for the welding of carbon steels, stainless/high alloy steels and work hardened steels.

Fairly sophisticated control systems carefully follow mandatory heating and cooling profiles, as outlined by welding code. Control systems with thermocouples that monitor internal furnace temperature, along with weldment part temperature are usually included. These control systems also include the ability to ramp up to temperature and cool off over specified time intervals. Finally, the use of recorders or data loggers is common for quality control documentation.

Additional benefits of pre and post weld heat treating are:

• Improved ductility and/or harness
• Lower risk of brittle fracture
• Improved metallurgical structure
• Reduction in diffusible hydrogen induced cracking 

Most pre or post weld heat treating furnaces are customized in one way or another, whether it size, total BTU output, energy source (electric, gas, oil), control scheme, door configuration, or having the furnace built on wheels or on rails.

Furnaces can be designed permanently, portable, or field erected. By having the furnace local, or right on-site, companies eliminate the need to send parts out and thus save time and costs associated with shipping. Depending on the scope of work, its not uncommon for the investment made in a fabricated furnace to pay off within the first year.

For more information on any custom built pre heat or post weld heat treat furnaces, contact:

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