Hot and Cold Working Processes (Chaps. 13, 14)

Hot Working Processes At high temperatures, metals weaken and become more ductile. Can do massive deformation of material. Important hot-working processes: (Most can be done cold as well.) Rolling Forging Extrusion Drawing Piercing

Rolling Definition: Process of reducing thickness or modifying the cross-section of a long workpiece by compressive forces applied by a set of opposing rolls. It can be a hot, warm and cold working process. Usually the first process in converting a cast material into a finished wrought product (sheets, plates, bars, strips).

Rolling Wrought products will, in turn, serve as input material such as cold forming or machining. First done at a high temperature to change ingot structure from coarse-grained/brittle/porous to wrought structure with finer grain size/better properties. Rolls are costly, therefore only standard shapes and sizes with sufficient demand are rolled.

Typical Shapes produced by Rolling Blooms: rectangular or square cross section; thickness t > 6 ”; width < 2 x t. Billets: circular or square cross section; smaller than bloom. Slabs: rectangular solid, w  2 x t. Can be further hot rolled into plates, sheets, strips. These in turn are cold-rolled into thinner materials (i.e. foil). Plates: rectangular solid: (1/4” < t < 12”) Typically used for structural applications. Sheets: less than ¼” thick. Shipped as flat pieces or coils depending on the industry. Used for auto bodies, appliances, office equipment, beverage containers.

The Rolling Process Heated metal passes between two rolls that rotate in opposite directions. Gap between rolls is less that material thickness entering. Rolls rotate at higher velocity than incoming metal. Friction propels the metal forward. It is then squeezed. It elongates as the cross section is reduced. See microstructure in Fig. 13-6.

The Rolling Process Important for metal to be uniformly heated to proper temperature before processing to produce uniform deformation. Soaking: prolonged exposure to elevated temperatures. Soaking Temp. for steels: ~ 2200 o F Not enough soaking, outside will flow better than inside. If cooled too quickly between rolling operations, a harder surface will resist deformation and promote cracking and tearing. Recrystallization at the proper temperatures ensures that the grain size produced is uniform and fine.

Roll Arrangements Rolls are arranged differently depending on application: (fig. 13-11) Two-high / three-high mills: used for roughing of cast ingots. Diameters (24” to 55”). Three-high: direction is reversed after each pass, where the plate is put through the upper roll gap, lowered and run through the lower roll gap.

Roll Arrangements Four high / cluster mills: smaller rolls lower roll forces, power requirement and spreading. Small rolls are supported by larger rolls since they will tend to deflect more under roll forces. Advantage: easier/cheaper to replace smaller rolls. Rolls: must have strength and wear resistance. Typically made of cast iron, steel or forged steel.

Ring Rolling Expansion of a thick ring into a larger diameter while reducing its cross section. Rolls are set closer together as they rotate. Reduction in thickness is compensated by increase in ring diameter. Typical applications include engine rings, rims, flanges, etc. Can do at room or elevated temperature. Advantages: short production times, material savings, good tolerances.

Ring Rolling

Rolling Quality Will produce directionality in properties. Not much in thick plates. High in thin sheets. Free of residual stresses could be induced with non-uniform cooling Usually covered with mill scale due to oxides; removable by pickling. Tolerances: 2-5% of dimension. (3/16”– 5/8” per ft) Surf. Finish: Average (1000 – 500 µin.)

Problems in Rolling Roll forces may deflect and flatten the rolls. The roll mechanism may stretch under roll forces. Roll forces can be reduced by: reducing friction using small diameter rolls to reduce contact area. take smaller reductions per pass to reduce contact area roll at higher temperatures to lower material strength.

Problems in Rolling Crowning: since roll forces tend to bend rolls elastically, strips will be thicker and display a crown or curved top/bottom. Can grind rolls so diameter at center is larger than at the edges.

Problems in Rolling Spreading: When the width increases too much within the roll gap. This is more likely in plates and sheets with small width to thickness ratios. Spreading also increases with increased friction, and smaller roll radius to strip thickness ratio. Can use vertical rolls at the edges of rolled product to constrain sides.

Defects in Rolled Plates / Sheets Recall that internal structural defects are undesirable. Surface defects (scale, rust, cracks) Wavy Edges (due to roll bending) where strip is thinner at the edges. Edge Cracks (due to poor material ductility at rolling temperature). Alligatoring (due to non-uniform deformation during rolling) or defects in billet.

Thread rolling Advantages Cold forming process Round rods are passed between reciprocating dies, forming straight or tapered threads. Advantages No scrap loss. Good strength due to cold work. Good surface finish. Induces compressive residual stresses at surface. Rolling does not cut through the grain-flow lines of the material, while machining does.

Rolling Mills

Forging

Forging “Controlled deformation of metal under pressure”. Family of processes where workpiece is shaped by localized compressive forces applied through dies and tools. One of the oldest metalworking operations. Require a set of dies and a power press, hammer or special machine.

Forging Forging vs. Rolling: Advantages Rolling produces continuous strips, plates, etc. Forging produces discrete parts. Advantages Can control flow and grain structure: better resulting strength and toughness. (see fig. 17-4) Good for high stress applications.

Forging Processes A metal may be: drawn out (drawing) (to increase length and decrease cross section) upset (upsetting) (to decrease length and increase cross section) squeezed in compression dies (to produce multi-directional flow)

Common Forging Operations 1. Open die Drop-hammer Forging 2. Closed / Impression Forging 3. Press Forging 4. Upset Forging 5. Automatic Forging 6. Roll Forging 7. Swaging

1. Open-Die Drop-Hammer (Impact) Forging Simplest forging process. Performed between flat dies with no precut profiles in the dies. Usually produces simple shapes with massive equipment. Parts generally weight 30–1000 lbs; can go up to 300 ton. Workpiece is placed between two flat dies and reduced in height by compression. Also called upsetting. (remember: flat plates).

Open-Die Forging

Open-Die Forging Barreling Part deforms uniformly in theory, but develops a barrel shape in practice. This is due to frictional forces at die-workpiece interface. Can reduce with lubrication.

Open-Die Forging Cogging Used to reduce bar thickness. Thickness is reduced by successive forging steps at specific intervals. Can do long bar sections with a small machine.

2. Closed / Impression Die Forging Two or more dies containing impressions of the part shape are brought together. Forging stock undergoes plastic deformation. Workpiece acquires shape of the die cavities. Metal flow is restricted by the die contours. Therefore, flow of metal is controlled.

Closed-Die Forging - Flash - When the dies are brought together, some material flows outward of cavity: this material is called flash. Flash will cool quicker and forms a pressure barrier. This encourages filling of die cavity. In proper closed-die forging, there should be no flash: “flashless forging.” Undersize blanks will cause die cavity underfill. Oversize blanks will cause excessive pressure / premature die failure.

Forging Preforming Operations Multiple dies may be used to gradually distribute material to the desired regions of a blank: Fullering: to distribute material away from an area. Edging: distributes material towards an area. Blocking: forming of part into its rough shape. Trimming: removing of flash squeezed out during forging.

Precision Forging A type of closed die forging. Also known as net-shape forging. Use special dies to produce greater accuracy. Requires bigger equipment due to higher forces required. Used when finer detail is required. Expensive.

3. Press Forging In hammer or impact forging, metal flows in response to the energy of the collision between hammer and workpiece. In larger parts, center regions may remain undeformed. Can use press forging which uses force and pressure to slowly squeeze the metal. More uniform flow is obtained. More time consuming: premature cooling may occur. May need to heat dies, reheat workpiece. Larger equipment is required (10K+ ton) (Fig. 18-12).

4. Upset Forging Increases diameter of material by compressing length. Can do hot or cold. Uses split dies with cavities and inserts to form parts. Typical parts: bolts, fasteners, couplings. Most widely used of all forging processes.

5. Automatic Hot Forging This automated process allows raw material to be fed on one end and have finished products on the other. Parts are usually solid/hollow, round/symmetrical, up to 7” diam. Steps: Hot rolled bar is heated with induction coils in 60 secs. Bar is descaled with rollers. Bar is sheared into blanks. Blanks are transferred through forging dies / operations until final product is obtained. High production rates, good tolerances. Need large quantities to justify production.

6. Roll Forging Reduction in thickness of a round or flat bar stock. Axles, leaf springs. Machine has two semicylindrical rolls. Heated bar is inserted between rolls. Rolls rotate, bar is shaped as it is rolled out towards the operator (see fig. 18-17). Can reinsert in several grooves in same die to gradually form.

7. Swaging Radial swaging /radial forging. Usually a cold process. Rod is subjected to radial impact forces by reciprocating dies, which act as hammers. Motion is achieve through rollers inside the cage, like a roller bearing. Workpiece is stationary; die rotates. Used for sizing (getting a component to final dimensions). Limited by length of the supporting mandrel.

Swaging Tube Swaging (fig. 14.17 a,b) Can produce piping with the use of a mandrel. Cams press on workpiece from all sides. Gun barrels made this way. Good tolerances. Limitation: part diameter. Requirement: need pre-existing cavity.

Other Forging Processes Heading (cold) A type of upsetting Performed at the end of a round rod. Goal: to produce a larger cross section at the end. Part examples: bold heads, screws, nails. Can be cold, warm or hot. Coining (cold) Closed-die forging process used for coins, jewelry. May need high pressures to produce fine detail, as much as 6 times the material strength.

Typical Components made by Forging Bolts Rivets Coins Connecting rods Transmission shafts and Gears Drive shafts Hammers Wrenches Heavy tanks Missiles APCs Shells Other heavy artillery

Typical Components made by Forging

Discrete Part Forging Process Summary (1) Cut / create a slug, billet or preform (a small piece of raw material to produce a discrete part). Heat slug in furnace /descale if necessary. (Hot forging only). Pre-heat / lubricate dies (Hot forging only). Forge in proper die sequence Block Forge (to rough shape the part, may need multiple dies)

Discrete Part Forging Process Summary (2) Forge in proper die sequence (cont.) Finish Forge (get part to final net shape) Trim (to remove excess flash) Deburr (to improve surface qualities) Heat Treat (to improve internal properties/relieve stresses) Inspect

Forging Mill