Module 3,4,5 Flashcards
What are the the types of Surface hardening?
Flame hardening, Carburizing, Nitriding, Carbon Nitriding
What Is Flame hardening?
Flame harding is a process where the surface is rapidly heated into austenite and immediately quenched
What percentage of carbon is needed for Flame hardening ?
0.4% to 0.5%
How is Quenching often achieved in Flame hardening ?
water spray
What does the surface produce when Quenched?
martensite
What is the maximum hardness After Flame hardening ?
HRC 65
What is the case depth after flame hardening
5mm
What does Carburizing introduce to surface hardening ?
Carbon
Explain the process of pack carburizing
A low carbon steel is packed into a heat resistant box with carbonaceous material and placed in a furnace at a temperature of 875 to 925 degrees C for 3- 72 hours.
What does the carbonaceous compound generate?
Carbon monoxide/ Carbon Rich gas
What is the purpose of the carbon rich gas?
The rich gas reacts with the metal surface where it releases the carbon. The carbon diffuses into austenite and increases the carbon content.
What happens after cooling in pack carburizing
After cooling, the part is austenitized again and then quenched hardened to produce martensite. It is then usually tempered. The core remains tough since it has low carbon ferrite.
What is the carbon content after cooling the Steel in pack carburizing
0.7 - 1.2%
What is the disadvantage of pack carburizing?
Pack carburizing is a lengthy and dirty process, not readily adaptable to continuous operation.
Explain gas carburizing?
parts are heated in a sealed furnace that contains a carbon rich gas (e.g., methane). Large irregular shaped parts may also be gas carburized.
What is the maximum hardness for carburizing?
HRC 65
What happens if a part is overheated (over tempered) in service?
it can simply be austenitized, quenched and tempered again to regain the hardness.
What does aCarburized Gear show?
A Carburized Gear shows a martensite case (white layer at surface) with a Pearlitic core.
What does Nitriding introduce to surface hardening ?
Nitriding introduces nitrogen atoms into the surface of steel.
Explain Nitriding
The nitrogen reacts with alloying elements to form ceramic particles.To nitride an alloy steel, it is placed in a sealed furnace at 500˚C (below A1) into which ammonia gas (NH3) is passed.
What is the Purpose of ammonia gas in Nitriding
The ammonia gas disassociates and individual nitrogen atoms diffuse into the surface of the alloy. The alloying elements combine with the nitrogen to produce extremely hard nitrides (ceramics). Nitrogen gas cannot be used (instead of the ammonia) since the N2 is too large to enter the steel.
What temperature is the process of nitriding use?
Less than 500 degrees C
What are the advantages of nitriding?
Low distortion since no martensite or quenching is required.
Highest hardness (up to HRC 85) due to the ceramic particles.
Hardness is retained at elevated temperatures (e.g., 500˚ C), which would temper other steels.
_________ is a combination of nitriding and carburizing.
Carbonitriding (or cyaniding)
What are the properties of Carbon nitriding
The steel part is case hardened by immersion in a bath of molten sodium cyanide (NaCN) or potassium cyanide (KCN) and heated with a torch to above the A1 for several minutes.
Both carbon and nitrogen are absorbed into surface from the cyanide. The steel part is normally quenched immediately and subsequently tempered to the desired toughness and hardness.
The cyanide salts are poisonous when mixed with acid, which often prompts manufacturers to find other alternatives. This method does, however, produce a shallow case depth relatively quickly (just a few minutes).
How does Stainless steel obtain corrosion resistance?
Stainless steels or corrosion resistant steels (CRES) obtain corrosion resistance by being able to form an adherent chromium oxide layer (passive layer) on the surface.
What is the chromium content in Stainless steel?
10.5% Cr
How do Stainless steels lose their corrosion resistance?
An environment not containing oxygen.
Elements are present which interfere with the oxide layer.
Chromium is not in solution so cannot react with the oxygen in the environment.
An environment that removes the oxide film (very high flow).
What are the three Primary types of Stainless Steels?
ferritic, austenitic, martensitic
What is the composition of Ferritic Stainless steel?
11 to 27% Chromium, 0.2% C max (low C)
What are the properties of Ferritic Stainless steel?
The Cr (which is BCC) stabilizes the BCC structure so that ferrite is present at all temperatures up to the melting point (i.e., this material is not allotropic so does not have a phase change).
The lack of allotropism makes these grades not quench hardenable.
As with other BCC lattice materials, ferritic stainless steels have low toughness at low temperatures (i.e., they have a T15).
What are the Applications of ferritic Stainless steel?
furnace parts, valves, cutlery, etc.
What is the composition of martensitic Stainless steel?
10.5 to 18% Cr, up to 1% C (high C)
What are the properties of martensitic Stainless steel?
The carbon counteracts the chromium and allows austenite to form at high temperatures. Carbon is an austenite stabilizer.
Due to the high carbon and chromium contents, these steels have high hardenability (are easily quench hardened to form martensite).
The combination of martensitic structure and the formation of chromium carbides make these grades very wear resistant and hard
A problem with these grades is maintaining a minimum of 10.5% Cr in solid solution to obtain the corrosion resistance (i.e., if the chromium is tied up with carbon to form carbides, it is not able to react with oxygen to form the protective oxide layer).
Martensitic stainless steels tend to have lower corrosion resistance than ferritic or austenitic stainless steels
What are the Applications of martensitic Stainless steel?
surgical tools, knives, valves, springs.
What is the composition of austenitic Stainless steel?
12% Cr (minimum) for corrosion resistance
7 to 22% Ni stabilizes the austenite phase at all temperatures. Higher Cr alloys needs more Ni in order to get a fully austenitic structure.
C must be as low as possible (most contain 0.08% C max. although some may contain up to 0.25% carbon for creep resistance).
The most common CRES is 18/8 (AISI 304) containing 18% Cr and 8% Ni. They are many variations of 304. For example, AISI 316 is 18/8 but also contains 2% Mo for extra chloride pitting resistance for surgical applications.
What are the properties of austenitic Stainless steel?
Highest general (uniform) corrosion resistance which makes it the most common of all stainless steels.
Double the cost of ferritic stainless steels due to the high nickel content.
The Ni and Cr give good strength from solid solution strengthening.
Very formable even though they are very strong (FCC).
Very tough at low temperatures (FCC so there is no T15).
Non-magnetic (FCC).
Prone to chloride SCC (stress corrosion cracking) as shown in Figure 11. Ferritic stainless steels are superior where chlorides are present.
Extremely high work hardening rates. The austenite is metastable at room temperature. When cold worked, the austenite can transform into hard martensite, in addition to normal work hardening. During machining (e.g., drilling) deep cuts must be used to cut below the work hardened surface from the previous pass. When cutting CRES with a band saw, damage to a single tooth can cause excessive work hardening of the CRES which causes a chain reaction when the next tooth dulls. With good band sawing practice, a blade will cleanly cleave the CRES. The band will cut quickly, accurately, quietly and lasts a long time.
What are the Applications of austenitic Stainless steel?
high corrosion applications such as high temperature or oxidizing acid storage. It is NOT suitable for reducing acids, such as H2SO4 or other low oxygen applications.
What are the other types of stainless steels ?
Precipitation Hardened Stainless Steels (PH CRES), Duplex Stainless Steels
What is the composition of Precipitation Hardened Stainless steel?
17% Cr, 7% Ni, and 4% Cu, where the copper is added to allow for precipitation hardening.
What are the properties of Precipitation Hardened Stainless steel?
High strength steels. The part can be machined in the soft condition and then precipitation hardened to increase the strength. These steels are not quench hardenable. Cu (or Al or Ti) forms extremely fine precipitates when the temperature is raised to approximately 500C (precipitation hardening). As the precipitates grow they strain (stretch) the space lattice and block movement of dislocations which increases the strength. The advantage of PH over martensitic stainless is that there is no quench which causes distortion. PH parts will have much better dimensional tolerances.
What are the Applications of Precipitation Hardened Stainless steel?
Aircraft parts, high strength valves.
What is a Duplex Stainless Steel?
Duplex Stainless Steels are a combination of ferritic and austenitic stainless steels.
What are the properties of Duplex Stainless steel?
There is sufficient nickel in these alloys to stabilize only half of the grain structure to austenite. The remaining structure is ferritic. This gives the superior general corrosion resistance of austenitic stainless with the stress corrosion cracking resistance of the ferritic. These alloys are somewhat less expensive than austenitic since the nickel content is lower. The alloy also has a higher yield strength which means the parts can be thinner (less metal) and lighter/cheaper.
In some reinforced concrete applications, duplex steel rebar can replace plain carbon steel to reduce overall lifecycle costs.
What isSensitization (carbide precipitation)?
Sensitization (carbide precipitation) refers to CRES becoming sensitive to corrosion. It can lead to intergranular corrosion or weld decay.
How does Sensitization occur ?
Sensitization normally occurs in the temperature range of 500 to 900˚C when carbon migrates to the grain boundaries and bonds with chromium. Since chromium is bonded to carbon it cannot bond with oxygen to form a passive layer.
Sensitization will occur during slow cooling of the weld bead, which then will corrode in service.
What temperature does sensitization occur
500° C to 900° C
How do we prevent sensitization?
Use a low carbon stainless steel (below 0.03%). At this level there is insufficient C available to form significant amounts of Cr carbides. The most common type of stainless steel is AISI 304L. The L indicates a low carbon version of 304.
Solution heat treat anneal. Apply a high temperature post weld heat treatment (1050 to 1125°C) to redissolve the chromium carbides and get the Cr back in solid solution followed by a rapid cool (1 minute to below 500 °C) to prevent them from precipitating again.
Use the stabilized grade of stainless steel. Elements are added to the stainless steel that is strong carbide formers (called stabilizers). Nb (AISI grade 347) or Ti (AISI grade 321) tie up the carbon and prevent Cr carbides from forming.
What is the common problem?
Pitting corrosion is a common problem where elements or surface deposits are present that interfere with the oxide layer. Chlorine is often present in most waters as a dissolved salt. Chlorine is more reactive than oxygen, and some Cl will replace the O on the chromium. Chromium chlorides are locally removed since they are soluble in water. This leaves an extremely active anode location surrounded by a very large cathodic region. A large cathode to anode ratio accelerates localized pitting. Surface deposits (dirt, rust, etc.) can also locally prevent oxygen from getting to the surface, which prevents a chromium oxide layer from forming. This will lead to pitting.
What is creep?
Creep is the slow continuous plastic flow of a material under load usually at high temperature. It is characterized by sliding of the grain boundaries past each other.
When is creep a problem?
it is a problem at temperatures above 550˚C for steels
What can Increase the creep strength?
The addition of carbide formers (such as Cr, w, or Mo) can increase the creep strength of steels, which can form stable carbide particles at the grain boundaries. These carbides lock the grain boundaries in position, preventing movement and creep.
____________ all contain high carbon contents for increased wear resistance.
Tool Steels
What are the grades of tool steels?
W Water hardening (low hardenability plain-carbon steel) is the least expensive. (e.g., W2)
O Oil hardening contain small amounts of alloying (e.g., Cr, Mo, V, etc.) to increase hardenability). (e.g., O1)
A Air hardening (higher alloy alloys air quench to be used for larger parts which minimize the potential for distortion and quench cracks). (e.g., A2)
M Molybdenum is added to form MoC particles at the grain boundaries. High speed steel (M2) is used for cutting tools to retain hardness at high temperatures (creep resistant)
T Tungsten is added to form WC particles at the grain boundaries. High speed steels used for cutting tools (creep resistant)
H Hot work steels. Carbide forming elements are added for high temperature creep strength applications (such as hot forging dies).
What are High Strength Low Alloy (HSLA) Steels used for?
HSLA steel (also known as micro-alloyed steel) are used for pipelines and structural applications.
Advantages and disadvantages of HSLA
HSLA steels are cheap (low alloy saves money), however they are tough (with reasonable strength and ductility) which makes them a good candidate for replacing some of the more expensive steels with higher alloying contents. Their alloying consists of low levels of carbon (approximately 0.03%-0.2%) along with additions of carbide and nitride formers such as Ti, Nb and V.
How to produce HSLA
1) Steel is formed in 7” to 10” thick slab from continuous casting;
2) the slabs then reheat in a furnace at approximately 1200°C which allows dissolving all alloying elements;
3) the heated slabs then are sent to a roughing mill (hot rolling). The mill gives several passes with big reduction in thickness (down to 1”-2” thickness). During this process the material is subjected to significant amount of deformation and recrystallization. The Ti, Nb and V additions are very important at this stage. They form carbides and nitrides during cooling, which prevents grain growth and keeps the grains small for further refinement.
4) Afterward, the material is sent to finishing mill (hot rolling) which gives several smaller reductions into the final thickness (depending on application, as thin as 1-2 mm). In the finishing mill stage, little deformation is expected and the process is done mainly for precision and consistency.
The next step in process is controlled cooling, in which water jetting on top and bottom of the material cools the skelp in a manner specific to the application. Cooling can be adjusted to change the microstructure and material properties of the final product. There are several banks of these water jets, and each can be changed individually to deliver more or less water for a faster or slower cooling rate.
Before cooling, the material is still above 1000°C. In the next step, the material is cooled down to the desired microstructure, then brought down to 400-600 °C most of the time. For some specific microstructures, the material can be cooled to as low as 150°C or as high as 750°C.
6) Towards the end of the process, the thinned metal sheets are coiled at temperature to allow some microstructural changes to take place after coiling (this is the main difference between the final cooling temperatures above). This process may take several hours for cooling to complete. This process is primarily used to allow some precipitates to form in a controlled manner, but occasionally used for proper microstructure changes.
7) Depending on the required cold work based on specific applications, some additional compression and work may be applied on the materials afterwards for additional increase in strength
Cryogenic Steels
Due to the ductile to brittle transition effect for BCC ferritic alloy steels (see module 4 and the Charpy impact test handout), the mechanical components made of alloyed steels cannot be used at temperatures lower than -101ºC (-150ºF). However, an addition of 8.5 to 9.5% nickel into these alloyed steel associated with a proper heat treatment, allows these steels to withstand temperatures as low as -196ºC(-320ºF). As a result, these steels become suitable for cryogenic applications such as process piping materials used in gas liquefaction units. Recent research on the nickel-added cryogenic steels has shown that two microstructural mechanisms are at work to enhance the low temperature performance of these steels (which is accomplished by decreasing the ductile-brittle transition temperature). The first mechanism is based on grain refinement and the second mechanism is based on the introduction of a small amountmetallurgically stable FCC austenite (5 to 15%) into the ferrite BCC structure. These modifications can be achieved by special heat treatment mechanisms such as quenched-tempering (Q&T) and also double-normalizing and tempering (NNT). Addition of some other elements such as Manganese can also introduce similar cryogenic effects into ferritic alloyed steels.
What does Dual-Phase (DP) Steels Consist of ?
Dual-phase steels microstructure consists of a ferritic matrix (85 to 95%) with addition of a secondary phase martensite formation (5 to 15%) within this matrix.
What are the advantage and disadvantage of DP steels?
Presence of hard martensitic phase within ferritic matrix provides a greater strength for dual-phase steels as compared to other common types of high strength low alloy (HSLA) steels. Also, due to the presence of high volume of ferrite, the dual-phase steels are expected to have higher degrees of ductility and formability.
A proper controlled cooling from the austenite phase or from another dual-phase steels (such as the ones with ferrite plus austenite structure) can produce ferritic-martensitic dual-phase steels. During the cooling process, some of austenite phase transform to ferrite before a fast cooling rate transforms the remaining austenite to martensite.
Generally speaking, ferritic-martensitic dual-phase steels exhibit great strength, good ductility (uniform elongation) and good fatigue resistance which makes them ideal candidates to be used in many automobile industry applications such as body panels, bumpers and rear rails and other parts designed for shock absorption.
What is the composition of DP Steels?
DP steels have low levels of carbon (0.06–0.15 wt.%), however the manganese content is higher in these steels (1.5-3.0%). Manganese is added to stabilize the austenite phase by solid solution strengthening in ferrite. Other elements such as Cr, Mo, V, Nb and Si are also added into these alloys for enhancing the mechanical and microstructural properties.
When will Transformation induced plasticity (TRIP) steels transform?
TRIP steels will transform plastically when under impact to provide higher combination of strength and ductility as compared to most of the commercially available alloyed steels.
What is the composition of TRIP Steels?
TRIP steel are mainly composed of ferrite plus some amounts of other phases such as martensite, bainite, and retained austenite. During forming or impact of a TRIP steel, the retained austenite transforms to martensite resulting in a higher work hardening rate that persists to higher strains.
What are the advantage and disadvantage of TRIP steels?
TRIP steels have higher carbon content than dual-phase steels. The higher carbon content in TRIP steels are added to stabilize the retained austenite phase at temperatures below the room temperature. The addition of silicon and aluminum helps with the formation of ferritic and bainitic microstructure. Aluminum and silicon allow sustaining the required amount of carbon within the retained austenite by preventing formation of iron-carbide precipitation in the bainitic phase.
During an impact, the transformation of retained austenite-to-martensite absorbs energy which enhances the ability of the TRIP steels to withstand mechanical shocks. After the impact, the bulk of material still will be made up of ferrite, bainite or martensite, but a portion (or all) of the retained austenite will be transformed to martensite. Due to these properties, TRIP steels are being used in shock absorbing applications, such as body panels and bracing members in the automotive industry. TRIP steels also have been used for military armor purposes
What do Twinning-induced plasticity, or TWIP steels possess?
Twinning-induced plasticity, or TWIP steels possess an excellent combination of strength and ductility as well as wear and corrosion resistance.
What are the advantage and disadvantage of TWIP steels?
TWIP steels have austenitic FCC microstructure with a low stacking fault energy (SFE) that allows deformation twinning (a type of directional slipping within crystal structure of a grain). The twinning increases the strength by enhancing hardening and increasing ductility. These steels have superior mechanical properties at room temperature with UTS greater than 800 MPa and % elongation exceeding 100% of original length at the failure point.
TWIP steels possess excellent high work-hardening characteristics which makes them ideal in applications where shock and impact is detrimental to the life of a mechanical component. Similar to TRIP steels, TWIP’s are mainly considered for shock absorbent applications such as car panels and bracing members.
What is the composition of TWIP Steels?
TWIP steels chemical composition contains less than 1 wt% C, less than 3 wt% Si or Al but they have very high manganese contents (20 to 35 wt% Mn).
What are Physical properties
Properties that do not change significantly and are said to be fixed depending on the composition of the material.
Plastic deformation, thermal treatments and small amounts of alloying have little effect on physical properties. Examples are density, melting point, stiffness (Elastic Modulus), thermal and electrical conductivity.
What are Mechanical properties
The response of a material to applied loads (or forces). These properties can change significantly for the same material depending on how it was processed (e.g. deformation and thermal treatments).
Examples are strength, ductility, toughness, hardness, creep and fatigue.
What are three primary ways that engineers can alter the mechanical properties of materials ?
- plastic deformation (hot and cold working)
- thermal treatment (heat treating)
- grade selection (alloying)
What are the types of Stresses?
Tensile Stress (Tension), Compressive Stress (Compression), Shearing Stress, Torsional stress (torsion)
Explain Tension
Tensile Stress (Tension) causes the material to be stretched.
