Handout 2: Steels Flashcards
When looking up the hardness of the centre of a steel under the CCT diagram ( in the data sheet), what is the corresponding hardness for the surface of the steel?
Surface hardness may be taken to be the value at the left hand end, for oil and water quenching (i.e rapid surface cooling rate). For air cooling, the surface and centre cooling rates are effectively the same - so the surface and centre hardness is the same.
Define critical diameter Do
The diameter of a bar, quenched in a given medium, which forms 50% martensite at its centre.
Describe the Jominy end-quench test
The Jominy end-quench test imposes a gradient of cooling rates on a single sample. A 100mm log by 25mm diameter bar is austenised, dropped into a rig and water-quenched on one end. After cooling, the hardness is measured along the bar length.
What are the effects of carbon content in hypo-eutectoid steels on hardenability?
Hardenability increases as C content increases (curves for bainite/pearlite move to the right) matertensite starts and finish temperatures fall with increasing C content (reflecting greater strain energy in higher C martensite)
What is the effect of austenite grain size on hardenability?
The grain size affects both the rate of nucleation of ferrite and pearlite, and the time taken for growth to complete the transformation.
The larger the austenite grain, the higher the hardenability.
Grain size can typically change the critical diameter by up to a factor of 10 this is comprable to the effect of changing C content from 0.1 to 0.8 wt%.
What is the effect of alloying on steels hardenability?
Substitutional alloying elements such as Mn, Ni, Cr, Mo diffuse slowly into iron, and delay the diffusional transformations from austenite to ferrite and pearlite.
The CCT curves more to the right, enabling martensite to form with lower cooling rates. The C-curves for pearlite and bainite formation also seperate in low alloy steels.
Explain tempering and its effects.
Martensite is softened and toughened by tempering. Tempering is reheating to a temperature below A1, to allow the supersaturated solution of carbon to precipitate as spheroidal Fe3C precipitates in a matrix of ferrite.
In alloy steels, alloy carbides also form during tempering - this is called secondary hardening.
What is the “carbon equivalent”?
A simple empirical measures of both hardenability and weldability is the “carbon equivalent”, CE:
CE = C + Mn/6 + (Cr + Mo + V)/5 + (Ni + Cu)/15
The higher the CE the higher the hardenability, hence CE provides a warning for loss of weldability. The normal upper limit for welding of steels without taking special precautions against embrittlement is typically CE = 0.45.
How do you find the equivalent diameter of steels for irregular shapes?
Irregular shapes must first be approximated to one of these shapes using a certain amount of judgement:
- first identifiy the bulkiest section (which has the lowest internal cooling rate)
- Note that cooling at that location is dominated by its smallest dimensions.
What are f and l in determining the equivalent diameter?
De = f . l
- l is the leading (smallest) dimension of the component at the location of interest (e.g. its radius or wall thickness)
- The factor f depends on:
- the geometry and dimensions of the body
- the heat transfer coefficient, h (i.e. the quenching medium)
Explain and describe how alloying steels increases cast properties.
Allooying is important in primary steelmaking before casting, to deal with residual impurities, e.g. adding Al to remove oxygen (preventing formation of porosity), or adding Mn to react with sulphur to prevent formation of brittle FeS.
Cast irons are inherently castable due to their high carbon content (giving lower melting temperatures). Their as-cast properties are enhanced by alloying, e.g. Si to give “grey cast iron” (Si promotes the formation of free graphite in cast iron, giving lower strength and better machinability), or adding Ce to give “SG cast iron” (Ce makes the graphite form spheres instead of flakes, improving toughness).
Explain and describe how alloying can increase weldability.
Microalloyed steels contain small alloying additions which form stable carbide particles (e.g. Ti to form TiC) to prevent grain growth during welding, and thereby reducing the possibility of forming martensite on cooling.
Explain and describe how alloying increases steels strength and toughness.
- Cr,Mn,Mo,Ni,Co and W all dissolve substitutionally, without special heat treatment. Solid solution hardening is retained at elevated temperature - this is exploited in tool steel and stainless steels.
- Ti,Nb,V,Mo,W and Cr all strongly form carbide precipitates - this is exploited in low alloy steels and tool steels. Metalworking temperatures are too high for plain carbon steels - the tool soften (“running the temper”). Temperatures can only be controlled using slow cutting speeds and a lot of coolant, at a higher cost.
Explain and describe how alloying can increase corrosion resistance in steels.
Stainless steels have high Cr content for corrosion resistance, in combination with other elements to give a wide range of mechanical properties.
Explain and describe how alloying can increase machiniability in steels.
Free-machining steels contain elements such as S that enhance machinability, by promoting the formation of inclusions. These are usually detrimental in steels due to their damaging effects on toughness and fatigue. The inclusions promote weakness in the cheap shear zone, reducing chip size and reducing cutting forces. The alloy constituents may also transfer to the tool cutting edge, acting as a tool lubricant.
