Study On The Effect Of Laser Surface Quenching On Fatigue Properties Of Materials

There are severe economic losses caused by the downhole tools’ fatigue during oil field’s production, so it is important to improve the fatigue performance of downhole tools. Laser surface quenching is a surface processing technology developed rapidly in recent years. It is already mature both in theoretical research and productive application after kinds of studies. Laser surface quenching heats the metal surface above the phase transformation point quickly, then the reverse transformation of austenite to martensite takes place rapidly as the material cooling itself. During this stage, the material’s inside structure remains unchanged so that we can obtain the material with good inside toughness and good surface hardness. Laser surface quenching has been used in many manufacture fields such as aerospace, automobile, metallurgy, abrasive tools and machinery producing, it also used to optimize the performance of oil field’s tools. Laser surface quenching can be used in many different fields and there are dozens of processing parameters which influence the quality of material. In order to meet the requirement of the supposed hardening effect, the effect law of every processing parameter must be studied.This paper has studied the influence of laser surface quenching on the fatigue performance of a downhole’s tool steel. The main processing parameters which influence the quenching quality are laser’s power, scanning speed and the spot size. The influences of the laser’s power and scanning speed on the quenching quality have been studied in detail after practical investigation. The CO2laser device has been used to processing the samples and the PQ-6rotating bending fatigue machine has been used to do the fatigue test. The test data has been analyzed with the mathematical statistics method and the S-N curves which show different fatigue limits of difference kinds of samples have obtained. The SEM has been used to find fracture’s differences in the crack initiation area and the expansion area and the final fracture zone. The metallographic microscope and micro-hardness tester has been used to study the microstructure and the thickness of the quench hardened layer. The test shows that the laser’s power has a great influence on the fatigue performance of the samples. There is no hardening effect when the laser’s power is too small or the quench hardened layer will be over-thick that the fatigue limit will be reduced when the laser’s power is too high. The laser’s power will be appropriate when the thickness of the quench hardened layer is about1.5mm. When the laser’s power are same, the scanning speed has a little effect on the steel’s fatigue performance, the hardening layers’ thickness decrease slowly as the scanning speed increasing.