When selecting filler metals for critical applications, the impact toughness of the weld metal is usually a very important consideration. By definition, impact toughness is the ability of a weld to absorb energy before fracturing when rapid impact energy is applied to the weld. In reality, this rapid impact energy can result from earthquakes, collisions, explosions and strong winds.
Why is Impact Toughness Important?
Filler metals with excellent impact toughness properties provide two benefits in critical applications found in oil and gas industries, shipbuilding etc. Firstly, the risk of brittle fractures in steel because of low service temperatures and impact loading is significantly reduced. Secondly, welds with high impact toughness help to arrest crack propagation so that emergency repairs can be conducted on time before catastrophic failure. Ideally, using the correct filler metal and following proper welding procedures should help prevent cracking altogether.
How do Filler Metals Provide Impact Toughness?
To formulate filler metals that provide excellent impact toughness, some elements are added while others need to be minimized. Nickel is one of the most popular elements that provides enhanced impact toughness in filler metals. Nickel alters the weld’s microstructure, thereby increasing its resistance to cracking. However, filler metals with more nickel tend to produce a sluggish weld pool and slightly more spatter. In addition, filler metal manufacturers also tend to reduce other elements such as phosphorous and sulphur since these elements have a negative impact on weld toughness. Ideally, filler metals with enhanced impact toughness tend to have no more than 0.03% of phosphorous and sulphur.
After formulation, the resulting weld metal must undergo a specific testing process to ensure a right balance is achieved. The Charpy V-notch (CVN) test is the most recognized method for measuring the impact toughness of a filler metal. A machined weld specimen is placed in a chilled bath to bring it to the desired test temperature. The specimen is then placed on a fixture and hammer-shaped pendulum is released, thereby applying an impact energy on the weld specimen. The pendulum breaks the weld specimen and the computer calculates the CVN impact value. This test is repeated with the remaining weld specimens and an average CVN value is obtained. These CVN values can be found on the filler metal data sheet and on the certificate of conformance.
How to Maintain Impact Toughness During Welding?
Besides selecting the correct filler metal based on the job requirement and qualifying appropriate welding procedures, there are also several factors in the welding operation that need to be considered in order to achieve the desired impact toughness.
If preheating is needed, as specified in the welding procedure specifications, then it should be adhered to. Preheating helps to slow down the cooling rate of the weld. This helps to maintain the appropriate microstructure and minimizes the risks of poor impact toughness. It is also important to keep to the required interpass temperature range and heat input range during the welding operation. A sufficiently high interpass temperature and heat input will help slow down the cooling rate and minimize cracking. However, an excessively high interpass temperature and heat input can result in a very large heat affected zone and poor impact toughness. Some welding procedure specifications may require post-weld heat treatment (PWHT) to be conducted to restore the material’s microstructure to a favourable state and restore its toughness. The specified duration and temperature range of the PWHT operations should be strictly adhered to.
Lastly, always use the shielding gas mixture as recommended by the filler metal manufacturer. Using different mixtures will change the mechanical and chemical properties of the weld metal, thereby negatively affecting the impact toughness.