We reveal common penetrant inspection misconceptions we’ve seen in the field in recent years...
Liquid Penetrant & Quench Cracks
What is Quenching?
Quenching is the soaking of a metal at a high temperature, followed by a rapid cooling process to obtain certain desirable material properties.¹
What are the Benefits of Quenching?
- Increases the metal’s durability by increasing it’s tensile strength
- It’s a relatively simple, and cost-effective way to enhance the metal’s toughness
- Quenching is relatively short in duration²
However, there are a few disadvantages of quenching. If not done properly quenching can lead to distortions and cracking.
How does cracking happen?
As the metal is plunged into the quenching media it cools rapidly, starting from the parts’ surface continuing to its center. As the metal cools, it’s volume increases; so the metals’ hardened surface has grown in volume as the metal’s center still remains softer in composition. As the center starts to cool it’s change in volume is now restricted by the already hardened surface layer. This causes internal stress that becomes placed on the part’s surface.
When the force from the internal stress becomes greater than the outer layer’s tensile strength, a crack forms.³
How are quench cracks found?
Enter liquid penetrant materials. NDT-grade liquid penetrant, with enough dwell time, will enter the crack; and with the successful use of NDT developer, it will draw the penetrant out and provide a contrast so the inspector can spot the indication successfully. For a step-by-step guide on the liquid penetrant process follow the below links:
¹ "Quenching" Corrosionpedia.com, 21 July 2020. https://www.corrosionpedia.com/definition/953/quenching
² Mishra, Pankaj. "What is Quenching Process And Why it is Necessary?" Mechanical Booster. https://www.mechanicalbooster.com/2018/09/what-is-quenching-process.html
³ Free, Miles. "Quench Cracks - 3 Ways to Recognize" Speaking of Precision. 10 September 2013. https://pmpaspeakingofprecision.com/2013/09/10/quench-cracks-3-ways-to-recognize/
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