Mechanical Properties Defined • Tensile strength is the maximum force required to produce failure. • Ductility refers to how much the material can stretch before it fractures. • Yield strength is the force required to cause a material to plastically deform or yield. • Heat input (kJ/in.) = amps volts 60 /1000 (travel speed in in./min) • Elongation is a measurement of a material’s ductility expressed in a percentage. Features of the Heat Input and Mechanical Properties Changes in heat input can cause significant variances in the ductility of a weld, as well as its tensile and yield strengths. A material’s strength and its ductility are related. As strength increases, ductility decreases, and vice versa. The general rule is that higher strength equals increased brittleness; however, higher strengths may be required in certain applications. The strength of the weld deposit increases with lower heat inputs. Using a lower heat input will generally result in smaller welds and requires more weld passes to fill the joint. As well as the changes in strength, lowering heat input will also reduce ductility, which can make the finished weld more susceptible to cracking. On the other hand, completing a weld with higher heat input results in larger weld deposits and requires fewer passes to fill a joint. This improves ductility and resistance to cracking but lowers tensile and yield strength — a disadvantage if the reduction is enough to cause the weld to fall below minimum requirements. As an example, an AWS E71T-1C or E71T-1M carbon steel wire, when used with a low heat input of 30 kJ/in., produces a tensile strength of 93,800 lb/in.2, a yield strength of 89,300 lb/in.2, and an elongation of 24%. Compare that to the same wire used with a high heat input of 80 kJ/in., which produces a tensile strength of 81,500 lb/in.2, a yield strength of 70,200 lb/in.2, and elongation of 29% — Table 1. There are pros and cons with each of the heat input options; the optimal choice depends on the application’s requirements. For the best results, consult the filler metal manufacturer’s recommended parameters for a specific product to help avoid issues caused by excessively high or low heat inputs. These recommendations suggest heat input ranges to produce the desired strength and ductility results. Shielding Gas Impact In addition to heat input, shielding gas selection affects the mechanical properties of a weld. There are some general factors to consider when using argon mixtures versus straight CO2 shielding gas. The scenarios are very similar to those regarding heat input variations, with the same relationship between strength and ductility. Shielding gas with higher argon content results in welds with higher tensile and yield strengths and lower ductility. Again, the higher strength may or may not be needed for the application, and the disadvantage is that the weld is more susceptible to cracking. Conversely, higher CO2 content in a shielding gas mixture improves ductility and crack resistance but lowers the tensile and yield strengths. As a result, the weld may fail minimum requirement standards if the numbers drop below necessary levels. Consider the different variances produced in this example: The same E71T-1C or E71T-1M wire mentioned previously used with 100% CO2 gas provides a tensile strength of 84,000 lb/in.2, a yield strength of 77,000 lb/in.2, and 28% elongation. The same wire used with a gas mixture of 75% argon/25% CO2 results in a tensile strength of 90,000 lb/in.2, a yield strength of 83,000 lb/in.2, and elongation of 26% — Table 2. There is more to consider when selecting shielding gas. Shielding gas selection factors in weldability, fume requirements, arc qualities, and more. The change in mechanical properties that shielding gas can cause, however, should always be considered, as it directly affects the weld quality. The Combination of Heat Input and Shielding Gas Because high heat input and CO2 can have a similar effect on mechani- Table 1 — Changes in Heat Input can Cause Significant Variances in the Ductility, Tensile, and Yield Strengths of a Weld Low Heat Input (30 kJ/in.) High Heat Input (80 kJ/in.) Difference Tensile 93,800 lb/in.2 81,500 lb/in.2 12,300 lb/in.2 Yield 89,300 lb/in.2 70,200 lb/in.2 19,100 lb/in.2 Elongation 24% 29% 5% Table 2 — Shielding Gas Selection Affects the Mechanical Properties of a Weld 100% CO2 75% Argon/25% CO2 Difference Tensile 84,000 lb/in.2 90,000 lb/in.2 6000 lb/in.2 Yield 77,000 lb/in.2 83,000 lb/in.2 6000 lb/in.2 Elongation 28% 26% 2% 44 WELDING JOURNAL / JULY 2016
Welding Journal | July 2016
To see the actual publication please follow the link above