Welding Metallurgy 2 - PowerPoint PPT Presentation

1 / 30
About This Presentation
Title:

Welding Metallurgy 2

Description:

Welding Metallurgy 2 – PowerPoint PPT presentation

Number of Views:783
Avg rating:3.0/5.0
Slides: 31
Provided by: ddi49
Category:

less

Transcript and Presenter's Notes

Title: Welding Metallurgy 2


1
Welding Metallurgy 2
2
Welding Metallurgy 2
  • Objectives
  • The region of the weld where liquid does not
    form
  • Mechanisms of structure and property changes
    associated with these regions

3
Heat Affected Zone Welding Concerns
4
Heat Affected Zone Welding Concerns
  • Changes in Structure Resulting
  • in Changes in Properties
  • Cold Cracking Due to Hydrogen

5
(No Transcript)
6
Look At Two Types of Alloy Systems
7
(No Transcript)
8
Cold Worked Alloy Without Allotropic
Transformation
Introductory Welding Metallurgy, AWS, 1979
9
  • Welding
  • Precipitation
  • Hardened Alloys
  • Without Allotropic
  • Phase Changes
  • Welded In
  • Full Hard Condition
  • Solution Annealed Condition

10
Annealed upon Cooling
11
Precipitation Hardened Alloy Welded in Full Hard
Condition
Introductory Welding Metallurgy, AWS, 1979
12
Precipitation Hardened Alloys Welded in
Solutioned Condition
Introductory Welding Metallurgy, AWS, 1979
13
(No Transcript)
14
Steel Alloys With Allotropic Transformation
Introductory Welding Metallurgy, AWS, 1979
15
(No Transcript)
16
Introductory Welding Metallurgy, AWS, 1979
17
Hydrogen Cracking
  • Hydrogen cracking, also called cold cracking,
    requires all three of these factors
  • Hydrogen
  • Stress
  • Susceptible microstructure (high hardness)
  • Occurs below 300C
  • Prevention by
  • Preheat slows down the cooling rate this can
    help avoid martensite formation and supplies heat
    to diffuse hydrogen out of the material
  • Low-hydrogen welding procedure

18
(No Transcript)
19
(No Transcript)
20
(No Transcript)
21
(No Transcript)
22
Why Preheat?
  • Preheat reduces the temperature differential
    between the weld region and the base metal
  • Reduces the cooling rate, which reduces the
    chance of forming martensite in steels
  • Reduces distortion and shrinkage stress
  • Reduces the danger of weld cracking
  • Allows hydrogen to escape

23
Using Preheat to Avoid Hydrogen Cracking
  • If the base material is preheated, heat flows
    more slowly out of the weld region
  • Slower cooling rates avoid martensite formation
  • Preheat allows hydrogen to diffuse from the metal

T base
Cooling rate µ (T - Tbase)
Cooling rate µ (T - Tbase)
T base
24
Interaction of Preheat and Composition
CE C Mn/6 (CrMoV)/5 (SiNiCu)/15
  • Carbon equivalent (CE) measures ability to form
    martensite, which is necessary for hydrogen
    cracking
  • CE lt 0.35 no preheat or postweld heat treatment
  • 0.35 lt CE lt 0.55 preheat
  • 0.55 lt CE preheat and postweld heat treatment
  • Preheat temp. depends on CE and plate thickness

25
Why Post-Weld Heat Treat?
  • The fast cooling rates associated with welding
    often produce martensite
  • During postweld heat treatment, martensite is
    tempered (transforms to ferrite and carbides)
  • Reduces hardness
  • Reduces strength
  • Increases ductility
  • Increases toughness
  • Residual stress is also reduced by the postweld
    heat treatment

26
Postweld Heat Treatment and Hydrogen Cracking
  • Postweld heat treatment ( 1200F) tempers any
    martensite that may have formed
  • Increase in ductility and toughness
  • Reduction in strength and hardness
  • Residual stress is decreased by postweld heat
    treatment
  • Rule of thumb hold at temperature for 1 hour per
    inch of plate thickness minimum hold of 30
    minutes

27
Base Metal Welding Concerns
28
Lamellar Tearing
  • Occurs in thick plate subjected to high
    transverse welding stress
  • Related to elongated non-metallic inclusions,
    sulfides and silicates, lying parallel to plate
    surface and producing regions of reduced
    ductility
  • Prevention by
  • Low sulfur steel
  • Specify minimum ductility levels in transverse
    direction
  • Avoid designs with heavy through-thickness
    direction stress

29
Multipass Welds
  • Heat from subsequent passes affects the structure
    and properties of previous passes
  • Tempering
  • Reheating to form austenite
  • Transformation from austenite upon cooling
  • Complex Microstructure

30
Multipass Welds
  • Exhibit a range of microstructures
  • Variation of mechanical properties across joint
  • Postweld heat treatment tempers the structure
  • Reduces property variations across the joint
Write a Comment
User Comments (0)
About PowerShow.com