alloysteels and heat resistant steels Flashcards
un alloyed steels
features
- low cost
- harmful elements as S P N O H
- Si Mn Al good for chemical reactions as de oxidizing
- C is the most important as determines the perlite and therefore the strenght
Strengthening mechanisms for non alloyed steels
- C and N interstitial atoms
- cold or warm working
- decrease of grain size
- Martensite as a constituents
- Precipitation hardening with particles with the smallest size possible
- Incorporating an another phase
Low C steels
microstructure
- Consits in harder perlite and softer ferrite,
- Increasing in cooling rate increase the perlite phase.
- increasign the cooling rate takes the benefits from harder phase and finer grains.
- Contain rolling induced banding perlite
- The ferrite and perlite bands lie side by side
- micro-cracks easily propagate
welding behaviour
- the Haz is estimated on CCt diagrams
- cause the heat and following cooling can be produced martensite and bainite
- grains of martensite are exposed to moltiaxial shrinkage stresses wich are the prerequisite of brittle fracture.
- CEV is an equation to estimate the cold cracking behaviour
- C in high quantity is a crack promoter so is limited to 0.25% for higher value of C there are pre-heating and slow cooling of the steels
- An other option is to act on the composition of the filler metal.
machinability
- hard phases rapid wear of tool
- non abrasive phases as chip breakers are usefull to lobricate and avoid tool wear for example MnS
higher strength steels grades
- alloying and heat treatment
- steels with hot working and cooling as thermo mechanical rolling
thermo-mechanical processing
(nelleslide abbiamo visto un processo particolare)
- Rapid coooling for finer bainite
- Small amounts of V Nb Ti to produce a fine dispersion of carbide, nitide precipitates (microalloying)
- Rolling process
- It is also possible to partially quench the steel by the support of a more intense water flow onto the product, the tempering phase is made by the inner metal .
multi phase steels
- two phase or more steels are used in car body maufacturing
- controlled holding at high temp at the end of rolling
- controlled cooling
structural steels for full heat treatments
- a soft annealed microstructure is required for machining operations
- The actual service properties can be improved by the quench & temp treatment of near net shape.
- the goal is to produce tough components that do not readily fracture
steels 0.25 - 0.5 C
- widely used in highly dynamically and cyclically loaded parts
steels 0.5-0.6% C
- high hardening capacity and low tempering temperatures produce stregth from 1300 to 2000 mpa with YS/UTS ration of 0.9, suitable for springs and torsion bars
maragin steels
-Annealing
- Q&T
- Aging
- applications: aerospaces,motors: high strength and hardness.
steels for high temperatures
Heat resistant steels need to have a stable microstructure during high temp. service.
- No martensitic
- Austenitic has a better resiteance to creep.
- Resistance to oxidation at high temp (Cr,Si,Al,Ni).
- Austenitic has the problem of adhesion of scale.
- Ferritic is prone to grain growth
Applications for heat resistant steels
Martensitic: not used couse not stable over tempering temp
Austenitic: worked and casted based on Cr, Ni are better than ferritic, but present larger thermal expansion that compromised scale adesion.
Ferritic: worked steels are alloyed with Cr Al Si to improved oxidation, Al is a problem in casting due to his inclusions. Prone to grain enlargment
Creep behaviour
The influence of hot enviroments promote creeps
- diffiuson of interstitial elements starts above room temp.
- dislocations are able to leave their slip planes
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