Materials Strengthening

Materials Strengthening

Metallurgy Background 3 • Learning Activities • View Slides; • Read Notes, • Listen to lecture • Do on-line workbook • Do homework • Lesson Objectives • When you finish this lesson you will understand: • Strengthening Mechanisms in materials • Processes for restoring ductility in strengthened materials Keywords: Solid Solution Strengthening, Transformation Strengthening, Grain Refinement, Deformation Strengthening, Precipitation Strengthening, Quench, Temper, Recovery, recrystalization, Solution heat treatment, aging, overaging

Materials Strengthening • Solid Solution Strengthening • Transformation Strengthening • Grain Refinement • Non-equilibrium Transformation Strengthening • Deformation Strengthening • Precipitation Strengthening

AWS Welding Handbook, 8th Ed Vol 1

Questions? • 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?

Steel Equilibrium Arrangement of Iron Atoms in Steel Ferrite Austenite Delta ferrite • 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

Time for iron atoms to rearrange and carbon atoms to diffuse Austenite 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

Weld cools too fast to allow atom diffusion and rearrangement Austenite Steel Austenite Transformation to Martensite • Increased carbon and alloy contents (AISI 4340) • Low heat inputs, thick sections give fast cooling rates Martensite

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 Steel Martensite: Good or Bad? BAD GOOD

Heat Treatment Of 1040

Linnert, Welding Metallurgy AWS, 1994

Maximum As Quenched Hardness

Tempering of Martensite

Questions? • 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?

Questions? • 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

Questions? • 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?

Homework

Materials Strengthening