materials strengthening

Materials Strengthening

Metallurgy Background 3

Lesson ObjectivesWhen you finish this lesson you will understand:• Strengthening Mechanisms in materials• Processes for restoring ductility in strengthened materials

Learning Activities1. View Slides; 2. Read Notes, 3. Listen to lecture4. Do on-line

workbook5. Do homework

Keywords:Solid Solution Strengthening, Transformation Strengthening, Grain Refinement, Deformation Strengthening, Precipitation Strengthening, Quench, Temper, Recovery, recrystalization, Solution heat treatment, aging, overaging

http://we019.eng.ohio-state.edu/we300/w300t/we300.htm

http://we019.eng.ohio-state.edu/wenc/wenc.cfm


• Solid Solution Strengthening

• Transformation Strengthening– Grain Refinement– Non-equilibrium Transformation Strengthening

• Deformation Strengthening

• Precipitation Strengthening

AWS Welding Handbook, 8th Ed Vol 1

Turn to the person sitting next to you and discuss (1 min.):• The BCC iron crystal structure is represented here. The “x” are the interstitial positions where the carbon atom might occupy. What percent expansion is necessary between the iron atoms to fit this carbon? •Phosphorous is about 40% larger than carbon. What do you think would happen if phosphorous was alloyed with the BCC iron?

Equilibrium Arrangement of Iron Atoms in Steel

• Ferrite - room temperature to 1670°F• Austenite - 1333°F up to 2715°F• Delta ferrite - 2353°F up to 2795°F• Exact temperatures depend on % carbon in the iron

Ferrite Austenite Delta ferrite

Steel

Austenite Transformation to Ferrite-Pearlite

• Low carbon and alloy contents (AISI 1020, ASTM A36)

• High heat input processes give slower cooling rates

Ferrite + Pearlite

0.1.1.5.1.T7.95.12

Austenite

Time for iron atoms to rearrange and

carbon atoms to diffuse

Steel

Austenite Transformation to Martensite

• Increased carbon and alloy contents (AISI 4340)

• Low heat inputs, thick sections give fast cooling rates

Weld cools too fast to allow atom

diffusion and rearrangement

AusteniteMartensite

Steel

Martensite: Good or Bad?

• High strength and hardness compared to ferrite + pearlite– Many steels are designed to

be quenched to form martensite then tempered to improve their ductility and toughness

• 4130• 4340

• A ferrite + pearlite base material might have hard, brittle martensite in the weld joint

• Hard, brittle martensite is a factor involved in hydrogen cracking

GOOD BAD

Steel

Heat TreatmentOf 1040

http://www.gatewaycoalition.org/materials_engineering/heattrt.HTM

Linnert, Welding MetallurgyAWS, 1994

Maximum As Quenched Hardness

Tempering of Martensite

Turn to the person sitting next to you and discuss (1 min.):• Often a pre-heat is required by codes to slow down the cooling rate from welding. What might happen to the as welded structure if a preheat of 400F were specified? 600F?

Turn to the person sitting next to you and discuss (1 min.):• When we weld a cold worked structure, the temperature in the heat affected zone near the weld metal is very high but as we move further and further from the weld, the peak temperature gets less until it reaches room temperature at some large distance. What might this temperature gradient across the HAZ do to the cold worked structure?

overaging

Turn to the person sitting next to you and discuss (1 min.):• Can precipitation hardening take place without first going through a solutionizing heat cycle?

materials strengthening

Documents

solution heat treatment

ferritepearlitelow carbon

martensiteincreased

carbon atom

materials processes

fdelta ferrite

brittle martensite

bcc iron crystal structure