BECOMING FAMILIAR WITH THE X-FACTOR

Ideal for use when welding boilers, process piping, heat exchangers and other applications subject to high service temperatures for long periods of time, this formula measures four trace elements that help determine a weld’s resistance to the brittleness that occurs when the weld is slowly cooled through a certain temperature range.

Good weld quality depends on having the right equipment, the proper filler metal and the best skills for the application. When welding chrome-moly steel (e.g. ASTM A387 Grades 11, 21, 22 and 91) for boilers, process piping, heat exchangers and other applications subject to high service temperatures for long periods of time, it also depends on being familiar with the X-factor.

Filler metal manufacturers tend to keep the X-factor low in filler metals for chrome-moly, but are not required to do so. For that reason, the more knowledgeable welding operators and supervisors can become about it, the better the chances of welding success on these materials.

The X-factor is a formula that helps determine a weld’s resistance to temper embrittlement, the brittleness that occurs when the weld is slowly cooled through a temperature range of approximately 850 deg F to 1100 deg F. It measures four trace elements that, combined, have the greatest impact on a weld’s susceptibility to temper embrittlement: phosphorous (P), antimony (Sb), tin (Sn) and arsenic (As). These trace elements are commonly found in the steel that comprises the outer strip of metal-cored and flux-cored wires, the core of stick electrodes and the steel used for solid wires. When held at high temperatures for a long period of time, these elements can weaken the structure of a weld, causing loss of ductility and toughness.

Measuring the X-factor can be done with a bit of simple math. Knowing the chemistry of the filler metal being used is the first step and can sometimes be determined by reviewing the product’s specification sheet. The formula to calculate the X-factor is as follows:

X = (10P + 5Sb + 4Sn + As)/100

In this equation, “X” refers to X-factor; “10P” is the amount of phosphorous recorded in parts per million (ppm) multiplied by 10; “5Sb” refers to the amount of antimony in ppm multiplied by 5; “4Sn” is the amount of tin in ppm multiplied by 4; and “As” refers to the amount of arsenic in ppm. After multiplying each of these items and adding them together, divide the sum by 100 to determine the filler metal X-factor.

The goal is to keep the X-factor at or below 15, in order to minimize the risk of temper embrittlement when welding chrome-moly steel. If the manufacturer does not list these trace elements on the product data sheet, or provide the X-factor number, contact your distributor or the manufacturer for this information.

FILLER METAL OPTIONS
Filler metals classified in the B product classes (e.g. B2, B3, B6, B8 and B9) work well for welding the grades of chrome-moly steel most prone to temper embrittlement. They typically contain a low X-factor. Such filler metals provide the chemical and mechanical properties needed to weld chrome-moly steel containing ranges of 1¼ percent chrome and ½ percent molybdenum to 10½ percent chrome and 1 percent molybdenum. In short, they are capable of withstanding high temperature service conditions without losing their toughness.

Filler metal manufacturers gain such attributes in their products by precisely monitoring and controlling all of the ingredients that go into them, particularly the steel. The goal is to select and use the purest steel possible. That is, steel that contains very low amounts of phosphorous, antimony, tin and arsenic.

Interestingly, in recent years there has been a trend toward refining ingredients in a way that provides filler metals with X-factor specifications lower than 15. This trend is due in part to the use of chrome-moly steels for increasingly higher service temperature applications and the presence of high-temperature service applications in colder regions. These applications benefit from having a lower X-factor, because it ensures better toughness and more resistance to rupture.

Remember, for these and other chrome-moly applications, never assume that the filler metal produces welds with a low X-factor number. Not all low alloy filler metals for welding chrome-moly steel have an X-factor below 15, nor are they required to meet this specification per AWS classification. Instead, rely on the X-factor calculation. And, if after calculating the X-factor for your given filler metal, it is above 15, contact your welding distributor or filler metal manufacturer for an alternative product.