Study-notes-for-test-5.docx

–Study notes for test 5 Tools Steels Tool Steels are hardenable carbon or alloy steels with a lot more alloying elements and use specialty secondary refining methods for cleanliness and alloy control. Higher alloying means: 1. 2. 3. 4.

Higher CE, and better and deeper hardness. Better heat resistance (hardness) Very difficult to machine due to high hardness. (annealed to shape) Very wear resistant carbide formations. Type 1-6 dispersion is good (pg 470) where as clumping and banding (types 7-9) are very bad. 5. Less severe quench means less distortion and cracking. 6. Some tool steels actually get harder as they heat up instead of softening. Tool steels are used to cut other metals because of their high hardness, but as production rates increase, so do cutting speeds, and even with cooling fluids, the tool bits can overheat and lose their strength and cutting edge. Alloys such as molybdenum, tungsten, chromium, and vanadium allow tool steels to work at even hotter temperatures and maintain their cutting edge.

Carbide Formation Tool steels gain much of their strength from uniform dispersion of carbides throughout the metal. Type 1 through 6 dispersions provide the best characteristics while type 7-9 are poor structures that make tool steels brittle and create stress concentrators that give rise to cracking.

Types of Tool Steels Tool steels are identified with letter and number designates. They include: Cold work – W, O, A, D Hot work – H Shock Resisting – S High Speed – M, T Mold Steels – P Special Purpose – L

Cold Work Tool Steels These are all quench grade tool steels. W (Water hardening) O (Oil Hardening) A (Air Hardening) D (High Chromium and carbon)

W – Series : Water hardened: This is a simple water quenched hardened alloy steel. They tend to have hard skins and soft cores, they are prone to cracking and spawling (scabbing) and lose their strength quickly when overheated. (Cheapest) O – Series: Oil Hardened: Has more alloying option, with a less severe quench and less distortion. A – Series: Air quenched: Has even greater alloy content, giving air hardenability up to 6” thick. D – Series: Above 12% Cr and 1.2% C. Widely used for it’s excellent wear characteristics, no cracking, minimal size change when heated.

Hot Work Tool Steels H – Series: These forms carbides that block dislocation movements and reduce heat softening. H1X – Chromium w/ Vanadium resists 430 oC H2X – Tungsten w/ chromium resists 620 oC H4X – Molybdenum w/ tungsten and vanadium

Shock Resisting Tool Steels S – Series: These have lower carbon content to survive impact usage meaning better toughness, moderately high hardness and wears a bit more.

High Speed Tool Steels These are used to maintain high cutting speeds, which generates a lot of friction and heat. They have very high hardness and the highest content of any tool steel. Can handle 540 oC M – Series: (Molybdenum) has high molybdenum as well as moderate tungsten, chromium, and vanadium. T – Series: (Tungsten) has tungsten as wells as moderate chromium, vanadium, and cobalt.

Mold Tool Steels P – Series: These are used in plastic injection molding dies and die tooling. The have excellent wear characteristics and maintain their shape. The P6 series is unusual from the others P series mold steels in that it is not quench hardenable. It is pressed into shape, then carburized to give it a hardened surface instead.

Tool Steel Properties When deciding to use tool steels, take into account their hardening and use characteristics. Issues with hardening include: Safety in hardening: Some tool steels simply crack, even without stress concentrators, use less severe quenches. Depth of hardening: tool steels have deep hardening avoiding softer cores. Size change in hardening: Some tool steels exhibit non-linear dimensional changes as they heat up, and this will affect machining tolerances.

Resistance to decarburization: the surface of some tool steels can lose its carbon if overheated in air, becoming locally softer and crack.

Issues With Use Resistance to heat softening: select a tool steel that will not lose their hardness form heat generated by cutting friction. Wear resistance and toughness: some tool steels are better than others (choose the right tool steel for the job) Machinability: some tool steels can be annealed for reshapening then quench hardening again. Some tool steels can’t do this and are essentially disposable.

Tool Steel Defects Manufacturing defects are internal defects and are stress concentrators leading to cracking. Avoid. User defects can be prevented with care and cautious use. Grinder burn and electrical discharge - weld sparking and over grinding can create spots of black untampered martensite which is hard and brittle and a stress concentrator leading to cracking.

Hydrogen and Neutron Embrittlement Hydrogen enters interstitially. Hydrogen can become embedded in some processes, wedging itself into the microstructure causing “cat eye” cracking.

Cast Irons and Powdered Metallurgy There are 3 traditional methods for making parts: machining, welding and casting. Each has their own benefits and drawbacks, but casting is typically chosen when extensive shaping is needed or cast alloy properties are desired.

Casting Casting is defined as making a shape by liquifying a metal and letting it solidify into a new shape in a mold. Casting methods include: Sand/ Graphite casting. Permanent Mold Die Casting Shell Molding Investment/ Loss Wax Casting Centrifugal Casting

Sand and Graphite Casting These castings are made using packed sand or polymer injected graphite as a pattern negative. The polymer tends to give better binding and surface finish. The cope (upper portion of mold) and drag (lower portion) are made by packing sand around a wooden form with sprue and riser plugs, then removing the forms and plugs. The cope and drag are placed face to face, leaving a cavity ready to be filled with molten metal.

Molten metal is poured into the sprue and fills the pattern. Excess metal, debris, and gases can escape up the riser (doesn’t always work.) The size of the sprue, gate, riser and geometry will determine size location and type of casting defects. The part cools and solidifies, the part is broken out of the sand (destroying the mold) and the riser and sprue are cut off to be recycled.

Drawbacks: Slow Process One time use (mold is destroyed) Rough surfaces (needs polishing) Limited sizes

Permanent Mold Casting Permanent mold casting uses a water or air-cooled mold, typically coated with a thin ceramic slurry to prevent abrasion damage. It is almost the same as sand casting except the mold is not destroyed. These tend to have better surface finishes, but also cost more. (3 for 1 redundancy required)

Die Casting These also use water cooled hydraulic dies that are closed together and metal (usually molten) is forced into the chamber under pressure. Certain low melting temperature metals can flow under pressure alone. The mold opens and the part is removed. Die casting has very accurate, complex structures with great surface details but has significant cost.

Shell Molding These use thermosetting resin applied to bond two metal shell halves together. The two bonded shells can be poured in and allowed to solidify. Fairly good details are possible. Poor mans permanent mold.

Investment/ Lost Wax A detailed wax replica is made and coated in layers of ceramic slurry and allowed to dry. The ceramic shell is then baked to melt out all the wax, leaving a hollow completed mold. Molten metal is then poured in and allowed to cool. The ceramic shell is then broken away.

Centrifugal Casting This process is sometimes used to make cast pipe. Molten metal is poured into a refractory lined rotating mold where centrifugal force pushes the metal to uniformly coat the walls to take the shape of the mold.

Casting Design Factors These factors must be considered before selecting a casting metal and method: Tolerances: different metals have different shrinkage rates, but all cast metals will shrink twice: once going from liquid to high temperature solid, and again as it cools to room temperature. As a result, all molds are made larger by a “shrinkage allowance” to account for the dimensional change. Surface Finish: die an investment casting give smoother surfaces where as shell and sand have the roughest and require machining after casting.

Size Limitations: fine and small castings should use die and investment casting. Sand casting can be used for larger castings but excessive size and weight may burst the sand mold (very bad) Casting Materials: Molybdenum and Tungsten simply can’t be cast because of their high melting points. Die casting steel is cost prohibitive, and many wrought alloys lack fluidity and don’t fill the cast completely. Casting Costs: Deciding which process (casting, welding, machining or 3d printing) to use relies on significant set-up costs so is usually best contracted out. If only one copy is to be made, then sand casting is the cheapest choice. Availability: lead time plays a crucial role in the manufacturing. Plan ahead! Sand casting has a one month lead time. Investment casting has a three-month lead time. Die/ Permanent has a six-month lead time.

Casting Design Be aware of how a casting will solidify: you will have columnar outer grains with possible equiaxed in the center. Grain direction greatly influences strength in certain directions. The outer grains will be fairly clear of porosity and segregation whereas the center may have more. Double shrinkage can lead to complex internal stresses leading to cracking so watch your shrinkage allowances. Casting geometry and layout need to be considered here. Mostly thick to thin transitions and uneven cooling lead to warping and cracking.

Types of Cast Iron The term “cast iron” typically refers to an iron – carbon – silicon alloy with greater than saturation limits of carbon content. Minor class of cast irons: Gray Iron Malleable Iron* Ductile Iron* White Iron Alloy Iron Be careful because these names a bit of a lie.

Gray Iron Gray Irons have 2-4% carbon and at least 1% silicon. The microstructure can be ferrite, pearlite or martensite. The carbon however is in the form of graphite blooms or flakes because of the silicon content. These graphite blooms affect properties: Damping capacity: absorbs motion. Electrical Resistivity increases. Magnetic characteristics Corrosion resistance: galvanic corrosion protection

Damping Capacity: the graphite suppresses vibration and elastic deformation making it ideal for machinery bases. Electrical Resistivity: graphite is electrically resistive and the coarser the more resistive it is. Magnetic Characteristics: all common gray irons are ferromagnetic (although austenitic grays are not) great for solenoids and dead man switches. Corrosion Resistance: black crystalline graphite doesn’t dissolve and forms a black glossy layer if the cast iron is attacked. Graphite is resistive and electrolytic corrosion needs current flow so cast irons resist corrosion fairly well, except in wet soils. This can be solved with polywrap and back current to stop corrosion.

Grey Iron Mechanical Properties Poor tensile properties – does not expand linearly due to graphite and is brittle. Try pushing a chain or pulling a brick. Some materials work better in tension then compression. Cast irons are great for compression, they suck at tension. Pour toughness and less ductility. Great compressive strengths. Porous structures ideal for lubricant impregnation for extended wear resistance. Must stress relieve before machining. Quenching requires long heating times so fast processes won’t work. Great for gears, cams and small machine parts. Welding causes cracking unless slow preheat and slow cooling allowed, uses specialty rod and temper afterwords.

Malleable Iron Malleable iron is an iron – carbon – silicon alloy with 2-3% carbon and 1-1.8% silicon and has significant ductility by using a very long thermal process to convert white iron into pearlite/ferrite and carbon nodules (called temper carbon.) This process takes a very hard and brittle white iron, and heat treats it at 1600-1800 oF for forty hours during the first stage of graphitization where it converts to austenite and temper carbon nodules. It is then held just under austenizing for 30 hours for a second stage of graphitization where it converts to ferrite and temper carbon nodules. It is then oil or air quenched. Malleable iron is tough and ductile can be loaded in tension. It is stable and machinable, and has good wear resistance. It can even be quench, flame or induction hardened. However, its extensive thermal conditioning makes it somewhat more expensive than grey cast iron.

Ductile Iron Ductile iron is a cast iron with spherical graphite nodules. It avoids the brittleness of grey and white iron, and the long heat treatments of malleable iron. It’s an iron-carbon-silicon alloy with 3-4% carbon and 23% nickel with some significant nickel deposits.

The spherical nodules are created by adding ferrosilicon “seeds” to start graphitization and control nodule size in the pearlite/ ferrite/ martensite matrix, giving better electrical conductivity and impact strength, and similar corrosion resistance to grey iron.

Austempered Ductile Iron ADI is austempering a cast iron resulting in coarse bainite structure. ADI has higher tensile strength and wear characteristics making it a favorite for cast metals. Be careful! Not all malleable and ductile cast irons are malleable and ductile. The name implies more malleable and ductile than a standard grey iron. May be brittle.

White Irons These cast irons get their name from their fracture appearance which is white compared to grey irons. The contain 2-4% carbon 0.5-2% silicon and about 0.5% manganese. White irons are made by rapidly cooling the cast or portions of the cast or portions of the cast using “chills” to remove the heat quickly. The portions are very brittle and wear resistant and utterly impossible to weld or repair.

Powder Metallurgy The concept takes fine mixed metal powders, compacts them into a shape, and heats them just enough to cause the metal particles to fuse together without melting then through the process of sintering. (can use tungsten and titanium which cannot be melted) These parts can be 30% porous and lighter and later vacuum impregnated with lubricants to make selflubricated parts. P/M parts are very competitive with casting or machining as away to make near finished parts with little waste. Aerospace and automobile manufacturers are very interested in these.

Sintering Powder metals are made by blowing high velocity argon into molten metal stream. The result is a spray of fine metal particles that are then quenched to solid state. The powders are blended for best compaction results. The powder is the pressed at 10-60 ksi into a weak cohesive state (“green”,) and then sintered together well under it’s melting temperature. This fuses the edges of the particles together giving interconnected porosity making it strong and light. Many metals use this process. If sufficient CE is available, P/M parts can be quench hardened, but avoid salt and oil quenches as these will become trapped. Diffusion treatments are preferred. As stated earlier, P/M parts can be exposed to a vacuum and then put into oil bathes to force lubricant into the part to create self-lubricating parts. Most P/M parts tend to be smaller in size to ensure compaction. Complex P/M parts can be made with metal injection molding (MIM) where the powder is mixed with wax binder that is burned off during sintering to give very high (~99%) density parts.

Many P/M parts are made with Hot Isostatic Processing (HIP) in which the powder is placed into compressible container and subjected to 20 ksi and up to 2000 oC in argon. This provides densification in all three axis and better overall strength. The compressible container is usually sacrificial thin stainless and after the container has been crushed it all axes and its dimensions have all a changed, it must be machined off. Small and large P/M parts can be made at costs that are competitive with forging, casting and machining. Investment casting is the main competitor for P/M parts. P/M parts require hardened steel does and these can be expensive. HIP made parts are also expensive due to equipment and process costs. P/M and investment castings offer the best surface finishes.

Factoids The blue paint in braveheart was called WOAD. It was actually tattoos from 600-800 years before braveheart; kilts came 300 years after braveheart. Vikings came to north America first becuause Leif Eriksons’ sister Freydis Ericksdottir was chased out for killing people. Road grinders have 100s of teeth to grind up the road so it can be remelted and put back down again. Recycled highways. Microwave could have been used on hot patch asphalt, but PETA had this prevented in the name of earthworms. 3 to 1 dies and subs: For every die/ sub in service, have one on standby and one being repaired. Or expect downtime. All animals are equal but some animals are more equal than others. A dead man switch is a solenoid used to deenergize a system. Cast iron fittings are making a comeback because they muffle sounds of flushing.