Process for producing crack-free nitride-hardened surface on titanium by laser beams

DETAILED DESCRIPTION Referring to FIG.
1, there is shown a microstructure of a laser nitride-hardened titanium substrate 1 having a base metal portion (not shown), and a hardened layer 3, forming the outer surface thereof.
Hardened layer 3, comprises a multiphase layer made up of large TiN particles 5 surrounded by a dendritic structure of dispersed smaller particles of TiN 7 in a matrix of titanium.
The large TiN particles are believed to be titanium nitride platelets and/or Type I alpha-stabilized particles.
The large particles are caused by an excess of nitrogen around the molten titanium, during nitriding.
These large particles are inherently brittle, and develop cracks 9 throughout the entire body of the particle.
The hardened layer thickness is in the range of 0.
005 to 0.
025 inches, as measured in a line perpendicular to the outer surface of the substrate.
Referring to FIG.
2, there is shown the same type substrate as in FIG.
1, but treated according to the process of this invention.
Base metal portion (not shown) has a hardened layer 23, forming the outer surface of the substrate.
Hardened layer 23 comprises a multiphase layer made up of a dendritic structure of very fine, well dispersed TiN particles in a titanium matrix.
This microstructure has the required minimum hardness of 52 Rockwell C.
There is a complete absence of any cracking of the hardened layer 23.
The relative size of the particles is shown by the gage length of 100 microns shown as 25.
FIG.
3 shows the view of FIG.
2 at magnification 1000.
The gage length 27 is 10 microns.
The improved microstructure is provided by the process of this invention.
According to the invention, a substrate of titanium is provided in the form of an object, such as a shaft for a pump.
The substrate is preheated in a furnace, in an atmosphere of air, to a temperature in the range of between 1000 and 1200 degrees F.
The rate of heating is not critical but can be in the range of 200 to 400 degrees F.
/hour.
Heating continues until the substrate is uniformly heated