LASER CLADDING OF SHIP COMPONENTS MICROSTRUCTURAL EVALUATION

1
LASER CLADDING OF SHIP COMPONENTS
MICROSTRUCTURAL EVALUATION

• 18 OCTOBER 2002
• BIJAYA ADAK
• DR. PHILIP NASH

2
Overview

• Basic Concepts of Microstructure in Clad Region
• Discussion on Clad Zone Microstructure and
  Solidification
• Discussion on Laser Cladding with / on Stainless
  Steel
• Discussion on Dependence of Melt Height of
  Substrate on Laser parameters.

3
Basic concepts The Clad Region can be divided
into three parts

• Clad material
• Interface
• Heat Affected Zone

4
Basic concepts The Overall Composition and
Microstructure of the Clad Material depends On

• Power density ( the laser beam power absorbed per
  unit area of the material surface).
• Interaction time.
• Cooling rate.
• Thermal Property of the Material.
• Degree of mixing due to conduction and convection.

5
Basic concepts Microstructure f (Rate of
Solidification)

• Once Laser Irradiation stops
• Rapid cooling occurs due to heat transfer to the
  bulk.
• Due to high solidification, resulting
  microstructure is fine and frequently contain
  non-equilibrium phases.
• Increased solid solubility occurs.
• As a result of the above microstructure, a
  variety of improved surface conditions with
  desired properties can be achieved. Foe example
  
• High Wear resistance
• High temperature oxidation resistance
• High Surface Hardness
• High Corrosion Resistance.

6
Basic concepts SOLIDIFICATION

• The Cooling Rate in Laser Cladding is very high
  (5 x 103 5 x 105 K/s).
• Hence, Solidification in the clad zone can be
  predicted by Rapid Solidification Theory.
• The Thermal Gradient is highest at the bottom of
  the melt pool and decreases quickly to a constant
  value.
• Local Solidification Rate increases as the
  surface is approached from the bottom of the melt
  pool. During most of the solidification it
  remains constant.
• The melt zone can be divided into three sub zones
  from solidification point of view
• Plane Front solidification
• Does not need nucleation.
• Proceeds unidirectionally from bottom to the
  top.
• Cellular Zone A very narrow Zone
• Dendritic Zone Most of the melt zone consists
  of dendritic Structure.

7
Laser Cladding with Stainless Steel

• Cladding carbon steel or low alloy steel by
  various grades of stainless steel is a method of
  producing materials with good corrosion
  resistance and mechanical strength at a
  relatively low cost.

8
Laser Cladding with Stainless Steel A coating
of AISI 316L S.S on a plate of 0.2 Carbon Steel
using powder injection technique.

• Microstructure consists of Austenitic dendritic
  structure with small volume fraction of
  interdendritic delta ferrite formed due to the
  segregation of Cr and Mo.
• Results
• Better Stress and Pitting Corrosion Cracking
  Resistance than bulk material
• In the overlapping area corrosion rate is high
  due to the presence of delta ferrite.

9
Laser Cladding with Stainless Steel UNS S44700
(Fe-30,Cr-4.4,Mo) used as clad material on
Carbon Steel

• Conventional Stainless steel (SS) has low
  resistance to Pitting and Crevice corrosion in
  Sea Water.
• Several Stainless steel have been developed by
  simultaneous addition of Molybdenum and Nitrogen
  to achieve higher resistance to above type of
  corrosion.
• Overcome by cladding UNS S44700 on Carbon Steel
• Result
• Homogeneous Clad Layer
• Dilution was negligible
• Highly Dendritic Structure
• Microstructure Consists Mainly Ferrite. Cr Mo
  segregate to the inter-denritic spaces.

10
Laser Cladding on Steel Cr, Mn and Carbon
powder used as clad material on AISI 1016 as
substrate.

• Exhibits better wear properties than Stellite.
• The microstructure was divided into two parts.
• A blocked shaped region- A very closely spaced
  carbide in ferrite matrix.carbides had hexagonal
  structure which was M7C3 type.
• A different type of carbide (M6C) was dispersed
  on the ferrite matrix with a very uniform and
  fine distribution.

11
Dependence of Melt Height on Laser parameter
Laser Cladding and alloying of a Ni-base
super-alloy on plain carbon steel

• One of the objectives in a laser cladding process
  is to decrease the melt height of the substrate
  as much as possible.
• Dependence of melt height on Laser parameters has
  been demostrated in this paper.
• Dilution factor Cact/Csup R/(DR)
• 1/(l 1)
• Where
• Cact Actual Concentration in alloyed / cladded
  layer
• Csup Concentration in untreated super alloy
• D Thickness of the substrate melted together
  with the super-alloy.
• l D / R
• R Initial thickness of the super-alloy sheet

12
Dependence of Melt Height on Laser parameter
. Contd.
When l ltlt 1 Composition of the molten pool
is same with untreated super alloyResults
Cladding l gtgt 1 Results Alloying

• Conclusion
• The impact of the Interaction time on the
  interface position is more than the impact of
  beam power.