dt1410 - materials and processes in design

DT1410 - Materials and Processes in DesignUNIT 4 – PROCESSING OF METALSJEOPARDYLABS.COM/PLAY/UNIT-4663

https://jeopardylabs.com/play/unit-4663

Hot Working of Metals

Hot Working:

Conventional wisdom says that “hot working” means that the metal is deformed or worked at an elevated temperature … however, technically “hot working” means that:

The metal is worked at a high enough temperature that no plastic deformation, strain hardening, or “cold working” takes place.

“Cold Working” discussed in Chapter 10


Hot Working – Formal Definition:

A process of forming metals while they are heated above the recrystallization or transformation temperatures.


Hot Working:

One way to check for hot working is to measure the hardness or strength of the metal before and after the working process; if no strengthening occurs, it is hot working

Hot working occurs such that before the metal cools, the temperature is high enough that is anneals the metal and removes the cold work that otherwise would have occurred


Hot Working Processes:

Hot Rolling – Forging

Swaging – Extrusion

Drawing – Spinning Piercing – Welded

pipe


Purpose of Hot Working During the ingot casting process (molten

steel being formed into useful shapes) steel cools from its surfaces toward the center

This forms large, columnar, dendritic grains

That is, the grains grow parallel to the direction of heat flow

These large grains are usually lower in strength and higher in elongation than is desired

Shrinkage voids also are formed which significantly reduce strength

Hot Working/Heat Treating

http://www.engineeringmotion.com/videos/306/heat-treating

http://www.engineeringmotion.com/videos/306/heat-treating


Purpose of Hot Working These undesirable grains and

internal shrinkage voids can be removed or altered by the processes of Hot Working:

Hot RollingDrawingExtrudingForging

Rolling Mill Intermediate Forms

Ingots are sufficiently plastic to be taken to a hot rolling mill to be shaped into one of the intermediate forms called:

Blooms 6” x 6” square semi-finished steels

Slabs 10” wide and 1.5” or more thick

Billets 1.5” by 1.5” squares

Rolling Mill Intermediate Forms

Blooms (and often ingots) are further processed in structural rolling mills into other structural shapes:

Railroad rails I-Beams Angles Wide-flange Beams Zees Tees

Steel Rolling

http://www.engineeringmotion.com/videos/397/steel-manufacturing-rolling





Recrystallization

Steel ingot, with its typical grain imperfections and voids, is unsatisfactory for applications where high strength is required.

Parts made from such steel would easily fail under impact loads

The “columnar” grains in a cast ingot must go through recrystallization to give steel the required strength.

Recrystallization

• A process in which the distorted grain structure of metals that are subjected to mechanical deformation is replaced by a new strain-free grain structure during annealing.

Anisotropy

A condition created by the rolling process that gives a material specific physical properties in different directions.

Rolled steel is strongest in the direction of rolling.

Forging

Forging Processes

Heating metals to forging temperatures greatly increases their plasticity and workability.

Because of this, hot forging was and still is a very useful method of forming steel articles, both large and small.

Forging Processes

Plasticity:

The quality of material such that it can be deformed without breaking.

Clay is a completely plastic material.

Metals exhibit plasticity in varying amounts.

Forging Processes

Forging:

A method of metalworking in which the metal is hammered into the desired shape, or is forced into a mold by pressure or hammering, usually after being heated to a plastic state.

Hot forging requires less force to form a given part than does cold forging, which is usually done at room temperature.

Forging Machines

Open-die Impression-die drop Press Swaging Upset See pages 178 – 188 in Bruce TB

Open Die Forging

http://www.engineeringmotion.com/videos/60/forging-open-die

http://www.engineeringmotion.com/videos/60/forging-open-die

Hot Extrusion

Extrusion: A process of forcing metal through a

die, similar to squeezing toothpaste out of a tube.

Usually under high pressure and at elevated temperatures, forcing materials through a die containing the shape desired so that a shaped product is produced.

Metal and polymer shapes are produced using this method.

Hot Extrusion Extrusion:

Stronger non-ferrous metals (aluminum, copper, brass, magnesium) and ferrous metals usually need to be at a forging temperature to make them plastic enough for extrusion.

Square and round tubular products, structural shapes (tees, zees, angles) and round, square, or hexagonal solid shapes are common extruded pieces

Hot Extrusion

Advantages of Extrusion:

Allows for the manufacture of intricate shapes

Example: Inexpensive bronze gears can be

extruded into 20 ft or 40 ft lengths and later cut off to the desired widths.

For short runs of product, extrusion is more economical due to lower tooling costs

Hot Extrusion

Two basic types of Extrusion:

Direct ExtrusionMetal placed in a

containerA Ram is pushedThe work billet is forced

to flow through one or more openings in a die.

Hot Extrusion

Two basic types of Extrusion:

Indirect ExtrusionA die is mounted into a ramAs the ram is pushed

against the work billet material flows through the die opening and through the ram

Extrusion

http://www.engineeringmotion.com/videos/415/metal-extrusion-manufacturing



Unit 4 – Manufacturing Processes --MetalsCHAPTER 10

COLD WORKING OF METALS

Cold Working of Metals

Historically, cold working was accomplished by hammering on soft metals (gold, silver, copper, etc) for jewelry and other ornamentation.

Today, cold-formed products range from very fine hypodermic needles to huge pipeline tubes and from tiny hair-sized filaments to propeller shafts for ships.

Almost any conceivable shape can be made from cold-forming processes.


Cold Working:

Deforming a metal plastically at a temperature below its lowest recrystallization temperature.

“Cold” working usually occurs at room temperature

Strain hardening occurs as a result of this permanent deformation.


Common methods of Cold Working:

Cold rolling – Blanking

Pressing – Drawing Forming – Extruding Bending –

Straightening Roll forming –

Spinning


Advantages of Cold Working: Better surface finish Closer dimensional tolerances Better machinability Superior mechanical

properties Better strength-to-weight

ratios Enhanced directional

properties


Disadvantages of Cold Working: Metal is more brittle Metal is less workable

• Annealing is required to continue the process

• The metal may contain residual stresses that can cause warping or distortion

• Subsequent heating (welding) will change the cold working structure and reduce its strength

Factors in Cold Working

Properties that enable metals to be “Cold Worked”

1. Plasticity: The ability of metals to be deformed permanently in any direction

without cracking or splitting.

The higher temperatures used in hot working make metals more plastic, but recrystallization at high temperatures prevents them from increasing in strength.

To achieve higher strengths and hardnesses, the metal must be deformed at normal temperatures (below the recrystallization temperature).


When metals are cold worked to a certain point, the next operation requires forces greater than those previously applied to deform the metal further.

Each operation brings the particular metal closer to its ultimate strength and point of rupture.

As the amount of cold work increases the yield strength it gets closer to the maximum stress or tensile strength, and the strain at failure decreases.

The objective in cold working, therefore, is to stop well short of failure.


The degree of deformation (amount of cold working) determines the level of toughness, strength, hardness, and remaining ductility.

If more deformation is needed, then a process anneal is used to restore plasticity.

Process anneal on the cold-worked steel is often carried out in a closed container of inert gas to avoid scaling problems – a technique called a bright anneal.



2. Ductility and Malleability: Ductility – the property of a metal that allows it to

deform permanently or to exhibit plasticity without rupture while under tension.

Steel, aluminum, gold, silver, nickel Malleability – the ability of a metal to deform

permanently when loaded in compression.

Metals that can be rolled into sheets or upset cold forged are malleable.

Most ductile metals are malleable, but some malleable metals are not very ductile (lead)



2. Ductility and Malleability:

Some metals and alloys such as stainless steel, high-manganese steel, titanium, and zirconium tend to work harden, that is, to quickly increase hardness as cold working progresses.

Work hardening is often a troublesome difficulty in machining operations.



3. Elastic Behavior: When a metal is placed under stress within its

elastic range it will return to its former shape when the load is removed.

If the metal takes on a permanent set by loading it beyond the elastic limit, it will be permanently deformed, but will “bounce back” to some extent because of its elastic properties.

This is known as “springback” or elastic recovery.



Elastic Behavior: The tendency for a metal part to return (or

relax) somewhat from the form to which it has been bent or deformed when the forces causing the change in the form are removed.


Elastic Properties of metals can also cause problems:

The vast majority of metals are “polycrystalline” – they consist of many crystals or grains that were initially formed when the metal solidified.

Cold working tends to stretch out the grain in the direction of working, yet the grains still may have atom lattices that are not aligned.

Residual stresses may be left behind after cold working.


Elastic Properties of metals can also cause problems:

Residual stresses:

Stresses induced within the structure of a metal by cold working, machining, and heat treatments and remaining in the metal after the treatment is completed.

These stresses are “alive” and active within the metal.

They can cause a correctly bored hole to be oblong or a straight piece of bar to “banana” when machined on one side.


Residual stresses: The solution to residual stresses are

to use a stress relief or recovery anneal

Stress Relief Anneal: The reduction of residual stress in a

metal part by heating it to a given temperature and holding it there for a suitable length of time. This treatment is used to relieve elastic stresses caused by welding, cold working, machining, casting, and quenching.

Cold Rolling in the Steel Mill

Hot-rolled steel bars and plates must be sufficiently over-size because the cold-finishing process reduces them in cross section.

The hot-rolled bars or sheets are placed in a hydrochloric or sulfuric acid dip that removes the scale – a process called pickling.

Cold rolling then flattens and lengthens the grains in the direction of rolling.

Blanking and Pressing

One of the most versatile forms of metal working is that of converting flat sheet and strip metals into useful articles.

Sheet metal can be:

PiercedPunchedBlanked

Blanking and Pressing

Piercing: Cutting (usually small) holes in sheet

metal. Punching:

The operation of cutting a hole in sheet metal using a die. The hole material is scrapped.

Blanking: The operation of cutting a shape with a

die from sheet metal stock. The hole material is saved and used for further operations.

Drawing, Forming, Extruding Metal

Stamping Operations:

Coining and Embossing are stamping operations that form the surface of metal.

Impressions of letters, figures, and patterns are formed by pressing them onto the metal


Stamping Operations:

Coining: Shaping a piece of metal in a mold or die,

often creating raised figures or numbers. Embossing: The raising of a pattern in relief on a metal

by means of a high pressure on a die plate.

Progressive Metal Stamping

http://www.engineeringmotion.com/videos/53/progressive-metal-stamping



Drawing, Forming, Extruding Metal Bar, Tube, and Wire

Drawing:

Bar Drawing: Figure 10.25 (p. 205)

Performed on a draw bench. The drawing process

hardens the metal and gives it a smooth finish.


Bar, Tube, and Wire Drawing:

Seamless Tubing: Figure 10.26 (p. 205)

Also cold drawn on a draw bench This process includes a mandrel

inside the tube to thin the walls and provide internal finish.

Drawing, Forming, Extruding Metal Bar, Tube, and Wire

Drawing:

Wire Drawing: Similar process to bar

drawing, but involves much smaller diameters of metal

Is a continuous process done on rotating equipment

dt1410 - materials and processes in design

Documents

processes of hot working

hot working of metalshot

working process

cold working

hot workingheat treatinghttp

rolling process

direction of rolling

high strength