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Sep 17, 2020

What is Impact Toughness?


When it comes to choosing filler metals for critical applications, it’s essential to find ones with the right mechanical and chemical properties. Having the right properties can help provide the proper impact toughness for the application, which is especially important when welding high-strength, low alloy materials. It is also important for applications subject to:Large ship docked on land surrounded by scaffolding

-Rapid loading
-Cyclic loading
-Low service temperatures
-Seismic activity

So, what is impact toughness and why is it important?

Impact toughness defined
By definition, impact toughness is the ability of a weld to permanently deform and absorb energy before fracturing, after applying rapid stress to it. Simply stated, it’s how much rapid impact energy a weld can take before it cracks. The application of the stress is typically under one second. In the real world, impacts can result from any number of events, such as:

-High winds
-Earthquakes
-Intentional and unintentional collisions
-Explosions

The value of proper impact toughness
Filler metals with proper impact toughness provide several benefits in critical applications found in structural steel, oil and gas industries, shipbuilding and more. These include:

1. Reducing the risk of brittle fractures in steel that are associated with the combination of impact and cyclic loading and loss of toughness at low service temperatures

2. Helping to arrest the propagation of a crack so emergency repairs can be made

Ideally, however, using a filler metal with good impact toughness and following proper welding procedures will help prevent cracking altogether.

Selecting a filler metal
Filler metal manufacturers follow strict standards for formulating their products, including those set forth by the American Welding Society (AWS) “A5” filler metal specifications. These specifications provide testing criteria for the filler metal, along with minimum impact toughness requirements for each filler metal classification. For general purpose applications, select a filler metal that provides minimum impact toughness properties that meet or are better than the impact toughness requirements of the application.

Selecting a filler metal for demand-critical welds in seismic zones is based on code requirements, such as AWS D1.8 Structural Welding Code – Seismic Supplement. Selection can also be at the discretion of the engineer designing the welding procedure and joint configuration. The goal is the same: to create a weld with the appropriate impact toughness to safely withstand stressors and impacts.

This article is the first in a three-part series about impact toughness. Read article two, How Do Filler Metals Provide Impact Toughness? and article three, How to Maintain Impact Toughness During Welding.