Heat Treatment and the Effect of Welding Week 2 Weld Decay

1
Heat Treatment and the Effect of Welding

• Week 2

2
Heat Treatment of Steels

• The basis of heat treatment is that FCC iron can
  dissolve all carbon in steel (up to 2 C), while
  BCC iron can dissolve practically none (lt0.02
  C).
• Steel heated until it is transformed completely
  to Austenite has all its carbon in solution
  uniformly distributed

3
Steel Phase Diagram
4
Contd

• On cooling the carbon will attempt to precipitate
  out of solution as Cementite
• By controlling the mode of cooling the
  distribution of Cementite hence the mechanical
  properties can be controlled
• Steels are heated slowly to the Austenite region
  ( 30 to 50 C) to ensure it is fully Austenitic
  that the grains are as small as possible
• Final properties depend on the mode of cooling

5
Cooling

• Annealing usually on cast hot worked steels
  with coarse grain structures to obtain grain
  refinement, stiffness ductility
• Particularly necessary on components requiring
  additional work
• Involves cooling slowly in the furnace or packed
  in sand

6
Contd

• Normalising air cooling from the soak
  temperature
• Gives maximum grain refinement consequently
  harder stronger steels
• Useful finishing treatment Pearlite formed is
  much finer than via annealing

7
Contd

• Hardening quenching into oil, water or brine
  from the soak temperature fast enough to prevent
  the formation of Pearlite
• New phase known as Martensite (supersaturated
  solution of carbon in ferrite) very hard as a
  result the steels become very brittle
• With water quenching the steel becomes too
  brittle for use becomes necessary to temper
  steel

8
Contd

• Tempering re-heating to the sub critical range
  (approx 650 C), where stresses set up on
  quenching are relieved, so reducing the
  brittleness
• Steel becomes tougher at the expense of hardness
• Quenching tempering are principally applied to
  high carbon steels, where high hardness is
  required or to alloy steels to achieve high
  strength

9
Welding

• Extensively used for joining materials together
• Very complex geometries can be effectively welded
• Produces cleaner lines and reduces painting costs
• Cheaper, simpler lighter than rivets or bolts
• The material is heated locally to its melting
  temperature
• Additional metal may be introduced and the joint
  is then allowed to cool naturally

10
Contd

• Allows greater freedom for design
• Allows for continuous beams girders
• Easy quick alterations
• Additions can easily be made

11
Methods Available

• Arc welding
• Gas welding
• Friction welding
• Spot welding
• Soldering
• Brazing
• Electron beam
• Laser
• Diffusion bonding

12
Ideal Requirements of Welding

• A.) Complete continuity should be maintained
  between parts to be joined
• Joint should be indistinguishable from the parent
  metal
• Practically the above is not always possible,
  although satisfactory weld performance can be
  achieved in most cases

13
Contd

• B.) The joining material should have properties
  that are similar to the parent metal
• Careful selection of welding rods etc. is
  therefore essential

14
Heat Affected Zone

• Weld is basically a rapidly formed casting
  surrounded by a heat affected zone (HAZ)
• A temperature gradient is set up in the material
  during welding
• Temperature gradient ranges from the melting
  point at the point of fusion to ambient
  temperature at some distance from the weld

15
Contd

• High temperature followed by fairly rapid cooling
  causes changes in the metallurgy of the metal and
  the joint quality can be affected by
• a.) Structure quality of the weld metal
• b.) Structure properties of the part of the
  metal in the heat affected zone

16
Rate of Cooling After Welding

• The slower the rate of cooling, the closer the
  structure to equilibrium
• Cooling occurs mainly by conduction in the parent
  metal, depending upon the thermal mass (thickness
  size of parent material)
• The greater the thermal mass, the faster the rate
  of cooling

17
Arc Welding

• The main method employed for structural steelwork
  is arc welding
• Principles Electrode or filler wire melts due
  to passage of welding current through the filler
  wire, Arc (plasma) back to the power source via
  the earthed component
• Typically arc temperature is 5000 to 30000K
• The melt is transferred across the arc several
  mechanisms droplets, spray etc.

18
Welding Process

• Basically require
• 1.) Heat source to effect fusion
• 2.) Satisfactory metallurgical properties
• 3.) An efficient process

19
Processes used

• Manual metal arc
• Automatic welding using continuous coated
  electrodes
• Submerged arc welding
• Carbon dioxide shielded metal arc (MIG)
• Electrostatic welding
• Stud welding

20
Typical Welds
Butt Weld
Full penetration
Partial penetration
21
Contd
Fillet Weld
t throat l1 vertical leg l2 horizontal leg
l1
t
l2
22
Defects

• Residual stresses
• Distortions
• Undercut
• Incomplete penetration
• Porosity
• Slag inclusion

23
Weld Metal Solidification Cracking

• Weld metal solidification cracking hot cracking
  longitudinal in a fillet weld blue appearance
  (oxidised surface) due to material composition
  and/or weld restrain bead shape

24
Heat Affected Zone (HAZ) Cracking

• Heat Affected Zone (HAZ) Cracking heat affected
  zone due to weld adjacent to bead affected by
  heat input cooling cycle depends on
  composition but cooling rate can affect
  microstructure hardening more brittle
  carbide formation
• Susceptibility also affected by hydrogen in the
  weld metal introduced from the weld rod which
  is consumable

25
Carbon Equivalent

• Metal arc welding of carbon carbon manganese
  steels need to be checked by reference to BS EN
  1011 2 2001 guidance on carbon equivalents
  suggests suitable preheat levels to reduce
  cooling rate for various thicknesses limits on
  hydrogen levels sometimes need post heat (heat
  treatment)

26
Empirical Formula
C Mn have a significant effect Cr, Mo, Ni, Cu
have little effect
Limited usually to CE value lt0.5
C carbon Mn manganese Cr chromium Mo
molybdenum V vanadium Ni nickel Cu
copper
27
HAZ Cracking
Weld bead
HAZ crack
HAZ
28
Lamellar Tearing

• Associated with non-metallic manganese
  sulphides silicates when rolled material is
  extended as planer type inclusions (like wrought
  iron)
• Welds run parallel to inclusions cracks are
  induced through contractile stressing across
  thickness of the plate

29
Lamellar Tear Diagram
Inclusions thin planer types
Lamellar tear
30
BS 4360 Steel (grade 50C)

• Typical ladle analysis
• C 0.21
• Mn 1.50
• Cr 0.025
• Mo 0.015
• Ni 0.04
• Cu 0.04

Determine the carbon equivalent comment on
weldability
31
Carbon Equivalent of BS 3460 Steel
32
Comments on Weldability

• Few problems are encountered at values lt0.25
• Higher values from 0.30 up to 0.70 may be
  tolerated if cooling is controlled precautions
  taken to keep down the hydrogen content of the
  weld the HAZ hydrogen can be introduced by
  moisture in fluxes tends to result in cold
  cracking unless dispersed by heat treatment

33
Contd

• If, of the elements in this formula, only carbon
  and manganese are stated on the mill sheet for
  carbon and carbon manganese steels, then 0,03
  should be added to the calculated value to allow
  for residual elements.
• Where steels of different carbon equivalent or
  grade are being joined, the higher carbon
  equivalent value should be used

34
Weld Decay in Stainless Steel
Weld
Heat Affected Zone
Region depleted of chromium no longer stainless
is attacked preferentially by corrosion
Grain boundaries (scale of grains grossly
exaggerated)
35
Welding Structural Steels

• Designed to be weldable
• No serious loss of performance in the weld or the
  HAZ
• Structural engineers make allowance for HAZ in
  the design process (typically a 20N/mm2 reduction
  in the yield strength is applied)

36
Electric Arc Welding
37
Electric Arc Welding Equipment
38
Use of Electric Arc Welding
39
Metal Arc Inert Gas Shielded
40
MIG Equipment
41
Use of MIG Equipment
42
Butt Weld
43
Slag Inclusion
44
X-Ray Testing