ENT 244 – Manufacturing Engineering Processes

Dr. Mohd Sani bin Mohamad Hashim

Chapter 2: Materials Selection and Heat Treatment

1

Materials Selection • The designer of any product, other than software

must get involved with material selection. • Only occasionally will the exact grade of material

be specified by the customer. • Even then the designer must understand the

material to be able to design the product.

2

Materials Selection

3

Materials Selection • The factors that need to be taken into account during

selecting materials for a components: 1. Material properties

• The expected level of performance from the material

2. Material cost and availability • Material must be priced appropriately (not cheap but right) • Material must be available (better to have multiple sources)

3. Processing Must consider how to make the part, for example: • Casting • Machining • Welding

4. Environment • The effect that the service environment has on the part • The effect the part has on the environment • The effect that processing has on the environment

4

Materials Selection • Four Basic Steps 1. Translation: express design requirements as

constraints and objectives

2. Screening: eliminate materials that cannot do the job

3. Ranking: find materials that best do the job

4. Supporting Info: handbooks, expert systems, web, etc.

5

Materials Selection 1. Translation • Function: What does the component do? • Objective: What essentials conditions must be met? • Constraints: What is to be maximized or minimized? • Free Variables: Identify which design variables are free?

6

Materials Selection 2. Screening • Methods to evaluate large range of materials • Material Bar Charts • Material Property Charts (density vs. Young’s Modulus) • Screen on Constraints • Rank on Objectives

• Young's modulus - Density • Young's Modulus - Cost • Strength - Density • Strength - Toughness • Strength - Elongation • Strength - Cost • Strength - Max service temperature • Specific stiffness - Specific strength • Electrical resistivity - Cost • Recycle Fraction - Cost • Energy content - Cost

7

Materials Selection 3. Ranking • What if multiple materials remain after screening? • Rank on Objectives • Objectives define performance metrics 4. Select, then verify with any supporting

materials

8

Heat Treatment of Steel • Heat Treatment - any process involving controlled

heating and cooling to develop certain desirable characteristics

• The temperatures at which this takes place are called critical temperatures

• Three components o Heat o Soak o Cool

9

Heat Treatment of Steel • Heat

o Above critical temperature ( for steel, this is around 760°C to 870°C)

o This erases stresses previously imparted into the metal • Soak

o Maintain high temp for a time period appropriate to the mass and thickness of the material

o This permits the molecules to blend and become homogenous; rearrangement of the internal structure of the steel occurs here

• Cool o Fast cooling makes steel hard; slow cooling makes steel

soft o Quenching – fast cooling by immersion in a liquid

10

Heat Treatment of Steel • Quenching media

o Brine o Water o Oil

• Slow Cooling o Air cooling o Furnace cooling o Sand pack

11

Heat Treatment of Steel • Common heat treatment processes

1. Hardening 2. Tempering 3. Stress relieving 4. Normalizing 5. Annealing 6. Case hardening

12

Heat Treatment of Steel • Hardening – add hardness to steel

o Heat – just above critical temperature o Soak o Quench - rapid cooling by immersion in a fluid such as brine,

water, or oil; then temper to relieve internal stresses

• Tempering (drawing) - chiefly reduces brittleness created by hardening; removes some hardness, and relieves strain to return the part to a usable state o Heat - heat to less than critical temperature (at least 100°C)

• lower temps - less hardness removed • higher temps - more hardness removed • 100°C - 200°C - tempers for hardness (strong, but brittle) • 200°C - 300°C - tempers for toughness (strong, but not brittle)

o Soak – based upon the mass of the material o Cool - in still air

13

Heat Treatment of Steel • Stress Relieving - a process to remove all hardness

(extreme tempering) o Heat - heat to below critical temp, 450°C - 550°C o Soak o Cool - in still air

• Normalizing - removing abnormal characteristics

and stresses from heat treating, welding, etc. o Heat - at least 40°C above critical temp o Soak o Cool - in still air, at room temperature

14

Heat Treatment of Steel • Annealing - a process to relieve internal stresses,

soften the metal, make it more ductile, and refine the grain structure - the opposite of hardening o Heat - to above critical temp o Soak - based on mass (1 hr per 25mm thickness) o Cool - extremely slowly (therefore softening) o Examples: furnace cooling, or packing the part in dry sand Brings a part back to “below normal” condition

15

Heat Treatment of Steel • Case Hardening - a process to create super

hard surface upon a malleable core • Two Methods:

1. Carburizing 2. Nitriding 1. Carburizing o Heat - use high temperature oven (~900°C) o Soak - for a short time (1-3 hrs), in a high carbon

environment o Cool - quench o Advantage - hardness runs deep (~1.8mm); good anti-

corrosion o Disadvantage - difficult to retain part’s tolerance after high

temperature heating (part often has to be re-machined) o Typical use: gears, cams, spockets

16

Heat Treatment of Steel 2. Nitriding o Heat - low temperature (~550°C) o Soak - long heat cycle (~ 30 hrs), in an ammonia gas

environment o Cool – in still air o Disadvantage - susceptible to corrosion, depth of

hardening is less (~ 0.1mm), dangerous (highly flammable) o Advantage - close tolerances (~ ± 0.025mm or 0.050mm) o Typical use: crankshafts, cylinders

17

Case Study – Material Selections • Form a group of 2-4 person. • Choose one object. • You need to change the material of the chosen object. • Report should consist of (but not limited to):

1. Original material category of the object 2. Translation 3. Screening 4. Ranking 5. Supporting Info(if any) 6. Conclusion

• Report should be hand-written • Submission: 2/3/2016 (3 – 4pm BKN6&7)

18