IE 337: Materials & Manufacturing Processes

IE 337: Materials & Manufacturing Processes

Lecture 11: Introduction to Metal Forming Operations

Chapters 6 & 18

2

This Time

Iron/Steel Production Overview of Metal Forming

Bulk Deformation Sheet Metal Forming

Process Classifications Hot Working Warm Working Cold Working Processes

Formability Properties Yield Strength Ductility

Iron and Steel Production

Iron making - iron is reduced from its ores Steel making – iron is then refined to obtain

desired purity and composition (alloying)

Iron Ores Required in Iron-making

The principal ore used in the production of iron and steel is hematite (Fe2O3)

Other iron ores include magnetite (Fe3O4), siderite (FeCO3), and limonite (Fe2O3‑xH2O, where x is typically around 1.5)

Iron ores contain from 50% to around 70% iron, depending on grade (hematite is almost 70% iron)

Scrap iron and steel are also widely used today as raw materials in iron‑ and steel making

Other Raw Materials in Iron-making

Coke (C) Supplies heat for chemical reactions and

produces carbon monoxide (CO) to reduce iron ore

Limestone (CaCO3)

Used as a flux to react with and remove impurities in molten iron as slag

Hot gases (CO, H2, CO2, H2O, N2, O2, and fuels) Used to burn coke

Iron‑making in a Blast Furnace

Blast furnace - a refractory‑lined chamber with a diameter of about 9 to 11 m (30 to 35 ft) at its widest and a height of 40 m (125 ft)

To produce iron, a charge of ore, coke, and limestone are dropped into the top of a blast furnace

Hot gases are forced into the lower part of the chamber at high rates to accomplish combustion and reduction of the iron

Figure 6.5

Cross‑section of iron-making blast furnace showing major components

Chemical Reactions in Iron-Making

Using hematite as the starting ore:

Fe2O3 + CO 2FeO + CO2

CO2 reacts with coke to form more CO:

CO2 + C (coke) 2CO

This accomplishes final reduction of FeO to iron:

FeO + CO Fe + CO2

Proportions of Raw Materials In Iron-Making

Approximately seven tons of raw materials are required to produce one ton of iron: 2.0 tons of iron ore 1.0 ton of coke 0.5 ton of limestone 3.5 tons of gases

A significant proportion of the byproducts are recycled

Iron from the Blast Furnace

Iron tapped from the blast furnace (called pig iron) contains over 4% C, plus other impurities: 0.3‑1.3% Si, 0.5‑2.0% Mn, 0.1‑1.0% P, and 0.02‑0.08% S

Further refinement is required for cast iron and steel A furnace called a cupola is commonly used

for converting pig iron into gray cast iron For steel, compositions must be more closely

controlled and impurities brought to much lower levels

Steel-making

Since the mid‑1800s, a number of processes have been developed for refining pig iron into steel

Today, the two most important processes are Basic oxygen furnace (BOF) Electric furnace

Both are used to produce carbon and alloy steels

Basic Oxygen Furnace (BOF)

Accounts for 70% of steel production in U.S Adaptation of the Bessemer converter

Bessemer process used air blown up through the molten pig iron to burn off impurities

BOF uses pure oxygen Typical BOF vessel is 5 m (16 ft) inside

diameter and can process 150 to 200 tons per heat Cycle time (tap‑to‑tap time) takes 45 min

Basic Oxygen Furnace

Figure 6.7 Basic oxygen furnace showing BOF vessel during processing of a heat.

Figure 6.8 BOF sequence : (1) charging of scrap and (2) pig iron, (3) blowing, (4) tapping the molten steel, (5) pouring off the slag.

Electric Arc Furnace

Accounts for 30% of steel production in U.S. Scrap iron and scrap steel are primary raw

materials Capacities commonly range between 25 and

100 tons per heat Complete melting requires about 2 hr;

tap‑to‑tap time is 4 hr Usually associated with production of alloy

steels, tool steels, and stainless steels Noted for better quality steel but higher cost

per ton, compared to BOF

Figure 6.9 Electric arc furnace for steelmaking.

Casting Processes in Steel-making

Steels produced by BOF or electric furnace are solidified for subsequent processing either as cast ingots or by continuous casting Casting of ingots – a discrete production

process Continuous casting – a semi-continuous

process

Casting of Ingots

Steel ingots = discrete castings weighing from less than one ton up to 300 tons (entire heat)

Molds made of high carbon iron, tapered at top or bottom for removal of solid casting

The mold is placed on a platform called a stool After solidification the mold is lifted, leaving

the casting on the stool

Ingot Mold

Figure 6.10 A big‑end‑down ingot mold typical of type used in steelmaking.

Continuous Casting

Continuous casting is widely applied in aluminum and copper production, but its most noteworthy application is steel-making

Dramatic productivity increases over ingot casting, which is a discrete process

For ingot casting, 10‑12 hr may be required for casting to solidify Continuous casting reduces solidification

time by an order of magnitude

Figure 6.11 Continuous casting. Steel is poured into tundish and flows into a water‑cooled continuous mold; it solidifies as it travels down in mold. Slab thickness is exaggerated for clarity.

22

Metal Forming

Large group of manufacturing processes in which plastic deformation is used to change the shape of metal workpieces

The tool, usually called a die, applies stresses that exceed yield strength of metal

The metal takes a shape determined by the geometry of the die

23

Metal Forming

24

Bulk Deformation Processes

Characterized by significant deformations and massive shape changes

"Bulk" refers to workparts with relatively low surface area‑to‑volume ratios

Starting work shapes include cylindrical billets and rectangular bars

25

Basic bulk deformation processes: (a) rolling

Rolling

26Basic bulk deformation processes: (b) forging

Forging

27Basic bulk deformation processes: (c) extrusion

Extrusion

28Basic bulk deformation processes: (d) wire/rod drawing

Wire/Rod Drawing

29

Sheet Metalworking

Forming and related operations performed on metal sheets, strips, and coils

High surface area‑to‑volume ratio of starting metal, which distinguishes these from bulk deformation

Often called pressworking because presses perform these operations Parts are called stampings Usual tooling: punch and die

30Basic sheet metalworking operations: (a) bending

Bending

31Basic sheet metalworking operations: (c) shearing

Shearing

32Basic sheet metalworking operations: (b) drawing

Drawing

200 µm wide channels

Microchannel Process Technology

channel header

channels

Single Lamina

• Channels – 200 µm wide; 100 µm deep

– 300 µm pitch

• Lamina (24” long x 12” wide)– ~1000 µchannels/lamina

– 300 µm thickness

Patterning: • machining (e.g. laser …) • forming (e.g. stamping …)• micromolding

Microchannel Process Technology

• Device (12” stack)~ 1000 laminae= 1 x 106 reactor µchannels

• Laminae (24” long x 12” wide)– ~1000 µchannels/lamina

– 300 µm thickness Bonding: • diffusion bonding• solder paste reflow• laser welding …

Patterning: • machining (e.g. laser …) • forming (e.g. stamping …)• micromolding

24”

12”

12”

12”

24”Cross-section of

Microchannel Array

Microchannel Process Technology

Bonding: • diffusion bonding• solder paste reflow• laser welding …

Interconnect• welding• brazing• tapping

24”12”

12”

Microchannel Reactor

Bank of Microchannel Reactors(9 x 106 microchannels)

• Device (12” stack)~ 1000 laminae= 1 x 106 reactor µchannels

• Laminae (24” long x 12” wide)– ~1000 µchannels/lamina

– 300 µm thickness

Microlamination [Paul et al. 1999, Ehrfeld et al. 2000*]

*W. Ehrfeld, V. Hessel, H. Löwe, Microreactors: New Technology for Modern Chemistry, Wiley-VCH,

2000.

24”12”

12”

Microchannel Reactor

Microlamination of Reactor

37

Formability Variables

Material Properties Recrystallization Temperature Ductility Fracture Resistance Strain Hardening Yield Strength / Elasticity

Process Variables Temperature Friction Lubrication Deformation Rates

38

Material Properties in Forming

Desirable material properties: Low yield strength and high ductility

These properties are affected by temperature: Ductility increases and yield strength decreases

when work temperature is raised

Other factors: Strain rate and friction

39

Typical engineering stress‑strain plot in a tensile test of a metal

Material Properties

40

Material Behavior in Metal Forming

Plastic region of stress-strain curve is primary interest. In plastic region, metal's behavior is expressed by the flow curve:

nK where, K = strength coefficient; and

n = strain hardening exponent • Stress and strain in flow curve are true stress and

true strain

41

Stresses in Metal Forming

Stresses to plastically deform the metal are usually compressive Examples: rolling, forging, extrusion

However, some forming processes Stretch the metal (tensile stresses) Others bend the metal (tensile and compressive) Still others apply shear stresses

42

Flow Stress

For most metals at room temperature, strength increases when deformed due to strain hardening

Flow stress = instantaneous value of stress required to continue deforming the material

where Yf = flow stress, that is, the yield strength as a function of strain

nf KY

43

Average Flow Stress

Determined by integrating the flow curve equation between zero and the final strain value defining the range of interest

where, = average flow stress; and

= maximum strain during deformation process

nK

Yn

f

1_

_

fY

44

Effect of Temperature on Properties

45

Temperature in Metal Forming

For any metal, K and n in the flow curve depend on temperature Both strength and strain hardening are reduced at

higher temperatures In addition, ductility is increased at higher

temperatures

46

Temperature in Metal Forming

Any deformation operation can be accomplished with lower forces and power at elevated temperature

Three temperature ranges in metal forming: Cold working Warm working Hot working

47

Hot Working vs. Cold Working

Hot Working:Deformation at temperatures above recrystallization temperature = 0.5 Tm on absolute scale

Less powerful equipment More isotropic properties Less residual stress Less strain-hardening

Ductility for deformation Easier secondary ops

Shorter tool life

Cold Working:Deformation performed at or slightly above room ambient temperature - no heating required

Less reactive environment Better surface finish Better dimensional control More anisotropic properties More strain-hardening

Strength for end-use Fatigue resistance

Warm Working: Performed at 0.3 - 0.5 Tm, - intermediate effects

Recrystallization and Grain Growth

Scanning electron micrograph taken using backscattered electrons, of a partly recrystallized Al-Zr alloy. The large defect-free recrystallized grains can be seen consuming the deformed cellular microstructure.

--------50µm-------

48

49

Cold Working Processes

Primarily Sheet Metal Working Primary Operations:

Shearing Bending Drawing

Primary Processes: Punching / Blanking Roll Bending Roll Forming Spinning

50

Strain Rate Sensitivity

Theoretically, a metal in hot working behaves like a perfectly plastic material, with strain hardening exponent n = 0 The metal should continue to flow at the same flow

stress, once that stress is reached However, an additional phenomenon occurs during

deformation, especially at elevated temperatures: Strain rate sensitivity

51

What is Strain Rate?

Strain rate in forming is directly related to speed of deformation v

Deformation speed v = velocity of the ram or other movement of the equipment

Strain rate is defined:

where = true strain rate; and h = instantaneous height of workpiece being deformed

hv

.

.

52

Effect of Strain Rate on Flow Stress

Flow stress is a function of temperature At hot working temperatures, flow stress also

depends on strain rate As strain rate increases, resistance to deformation

increases This effect is known as strain‑rate sensitivity

53

(a) Effect of strain rate on flow stress at elevated work temperature. (b) Same relationship plotted on log‑log coordinates

54

Strain Rate Sensitivity Equation

where,

C = strength constant (similar but not equal to strength coefficient in flow curve equation), and m = strain‑rate sensitivity exponent

mf CY

55

The constant C, indicated by the intersection of each plot with the vertical dashed line at strain rate = 1.0, decreases, and m (slope of each plot) increases with increasing temperature

Effect of Temperature on Flow Stress

56

Strain Rate Sensitivity

Increasing temperature decreases C, increases m At room temperature, effect of strain rate is almost

negligible Flow curve is a good representation of material behavior

As temperature increases, strain rate becomes increasingly important in determining flow stress

57

Friction in Metal Forming

In most metal forming processes, friction is undesirable: Metal flow is retarded Forces and power are increased Wears tooling faster

Friction and tool wear are more severe in hot working

58

Lubrication in Metal Forming

Metalworking lubricants are applied to tool‑work interface in many forming operations to reduce harmful effects of friction

Benefits: Reduced sticking, forces, power, tool wear Better surface finish Removes heat from the tooling

59

This Time

Iron/Steel Production Overview of Metal Forming

Bulk Deformation Sheet Metal Forming

Process Classifications Hot Working Warm Working Cold Working Processes

Formability Properties Yield Strength Ductility

60

Next Time

Sheet Metal Forming Analysis

Chapter 20