Presentation on theme: "Welding Metallurgy 2. Lesson Objectives When you finish this lesson you will understand: The various region of the weld where liquid does not form Mechanisms."— Presentation transcript:
Lesson Objectives When you finish this lesson you will understand: The various region of the weld where liquid does not form Mechanisms of structure and property changes associated with these regions Learning Activities 1.View Slides; 2.Read Notes, 3.Listen to lecture 4.Do on-line workbook 5.Do homework Keywords: Heat affected zone, Base metal, Solutionizing treatment, Aging, welding procedure, heat input, Hydrogen cracking, Carbon equivalent, Lamellar Tearing, Reheat Cracking, Knife-line attack,
Turn to the person sitting next to you and discuss (1 min.): Precipitation hardened austenitic stainless steel is used for high strength applications like rocket components etc. Reviewing the various procedures for welding precipitation hardened steels, what procedure would you recommend? Does it make any difference that this is austenitic stainless steel and not just plain carbon steel?
Turn to the person sitting next to you and discuss (1 min.): As we saw, the cooling rate can depend upon the preheat and the heat input. Many codes actually specify the range of heat inputs that can be used to weld certain materials. We had an equation to determine the heat input before. What is it? What processes have the highest Heat Inputs? The lowest?
Hydrogen Cracking Hydrogen cracking, also called cold cracking, requires all three of these factors –Hydrogen –Stress –Susceptible microstructure (high hardness) Occurs below 300°C 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 Cracking in Welds 0.1.1.5.2.T12.95.12
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 Carbon and Low-Alloy Steels 0.1.1.5.1.T9.95.12
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 Cooling rate T - T base ) 2 Steel Cooling rate T - T base ) 3 T base
Interaction of Preheat and Composition Carbon equivalent (CE) measures ability to form martensite, which is necessary for hydrogen cracking –CE < 0.35no preheat or postweld heat treatment –0.35 < CE < 0.55preheat –0.55 < CEpreheat and postweld heat treatment Preheat temp. as CE and plate thickness CE = %C + %Mn/6 + %(Cr+Mo+V)/5 + %(Si+Ni+Cu)/15 Steel
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 Carbon and Low-Alloy Steels 0.1.1.5.1.T10.95.12
Postweld Heat Treatment and Hydrogen Cracking Postweld heat treatment (~ 1200°F) 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 Steel
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 Cracking in Welds 0.1.1.5.2.T14.95.12
Improve Cleanliness Improve through thickness properties Buttering
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 Carbon and Low-Alloy Steels 0.1.1.5.1.T11.95.12
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 Steel
Reheat Cracking Mo-V and Mo-B steels susceptible Due to high temperature embrittlement of the heat-affected zone and the presence of residual stress Coarse-grained region near fusion line most susceptible Prevention by –Low heat input welding –Intermediate stress relief of partially completed welds –Design to avoid high restraint –Restrict vanadium additions to 0.1% in steels –Dress the weld toe region to remove possible areas of stress concentration Cracking in Welds 0.1.1.5.2.T15.95.12
Knife-Line Attack in the HAZ Cr 23 C 6 precipitate in HAZ –Band where peak temperature is 800- 1600°F Can occur even in stabilized grades –Peak temperature dissolves titanium carbides –Cooling rate doesn’t allow them to form again Weld HAZ Knife-line attack Stainless Steel