SURFACE HARDENING APPLICATIONS USING LASER BEAM

Introduction

Laser hardening is an effective surface hardening heat treatment process commonly applied to Fe-C alloys containing over 0.2% carbon in ferrous materials such as steel alloys and cast irons. A high-energy-density laser beam is applied to the material to be surface hardened, and the process is carried out at a temperature below the melting point of the material on the outer surface (900-1400°C). A temperature difference is created between the material's surface and its core region. Once the desired working temperature is reached, the torch generating the laser beam moves in a controlled manner at constant speed and distance across the surface. As the laser beam progresses across the surface, the surface is homogeneously and continuously heated to the appropriate temperature (austenization temperature). Under the influence of high temperature, iron atoms change their positions in the crystal structure. This process is called austenization. As the laser beam passes over the surface, the heated region cools rapidly on its own. No additional cooling medium such as air, water or oil is required. For this reason, it is an environmentally friendly application. The rapid cooling is of such a degree that it prevents the crystal structure from returning to its original state, resulting in the formation of a very hard crystal structure called martensite. Martensitic transformation (austenite-martensite) achieves high surface hardness (>55 HRc). Laser hardening is performed only along the outer surface of the workpiece. The hardening depth is generally in the range of 0.1-1.5 mm. In some materials, depending on alloy composition, this can be 2.5 mm or greater. Greater hardening depth requires a material of larger volume (greater wall thickness) to ensure that heat is dispersed rapidly and the hardening zone cools sufficiently quickly. Depending on the width of the laser beam source, it can be applied to large surfaces. The area to which the laser beam is applied advances across the surface in a rectangular geometry (band or stripe). Using various optical accessories of different sizes, the laser beam can scan an area up to 60 mm in width.

Laser hardening can be applied to the following alloy groups:

• Cold work tool steels, • Hot work tool steels, • Plastic mold die steels, • General plain carbon, unalloyed steels, • Case hardening steels, • Structural steels, • Cast steels, • Cast irons (grey, spheroidal graphite). The hardening depth is between 0.1-2 mm and the hardness values obtained are in the range of 35-68 HRC (depending on the chemical composition of the material).

Laser Hardening Process

Laser hardening operations are conducted in the power range of 1-4 KW. The power applied per unit surface is between 10³-10⁴ W/cm². The hardening width can be varied between 5-60 mm through optical accessories. The hardening depth can be achieved up to 2 mm depending on application speed and duration.

Laser hardening process

  Robotic control is of considerable importance in laser hardening operations for precision surface treatments. With the aid of positioners, the finest details can be reached on circular or angular surfaces. With CAD/CAM Toplas 3D® software, there is a rapid and effective surface treatment capability based on three-dimensional technical drawing data (Uniquetech).

Laser hardening operations are performed with robotic control.

Laser Hardening from a Metallurgical Perspective

From a metallurgical standpoint, laser hardening is the process of transformation from the austenite phase to the martensite phase as a result of localized surface heating at high rates with high laser beam energy followed by rapid self-cooling. The heating rate is extremely high at 1000 K/s, and homogeneous austenization takes place in the form of a thin layer on the surface. Following this, the rapid cooling effect transforms the structure completely to martensite (at temperature T1 as shown in the TTT diagram). As a result, the highest hardness is achieved homogeneously in the central region along the laser torch application axis.

Austenite to martensite phase transformation

Hardness Values Achieved in Laser-Hardened Alloys

Laser hardening is used locally to increase wear resistance and service life of workpieces in a wide variety of applications, from forming dies to drilling equipment. This technology can particularly be applied to complex-shaped parts, hollow parts, and edge surfaces with minimal heat input. These applications are generally performed on ferrous alloys and cast irons that can be hardened by heat treatment.

Process Advantages:

• Precise and focused heat input can be provided. • High wear resistance performance and hardness can be achieved. • No additional cooling requirement. • Low loss of ductility. • Post-process additional treatment requirement is minimal. • No distortion occurs in the workpiece. Dr. Ekrem Altuncu ETSA / TESLAB Faculty of Technology Department of Metallurgy and Materials Engineering Sakarya University     Osman Kırlı Partner - Director Uniquetech Engineering  

Laser Hardening Applications

Cutting surfaces and edges subjected to laser hardening

Forming dies subjected to laser hardening

Through laser hardening heat treatment, a 4140 shaft gains hardness from 28-32 HRC to 58-60 HRC

Example applications

References [1] http://uniquetech.com.tr/ [2] http://www.us.trumpf.com/ [3] http://www.nutech.de/ [4] http://www.erlas.de/ [5] http://www.lasercladdingservices.com.au/ [6] www.laserline.de [7]www.cla.fraunhofer.org/en/laser_heat_treatment/ [8] http://www.lasertherm.com/ [9] https://groupsixtech.com/ [10] Laser Material Processing 4th ed. 2010 Edition, by William M. Steen (Author), Jyotirmoy Mazumder (Author), Kenneth G. Watkins.