Cold Worked Tool Steels Flashcards
What are the maximum temperatures for their operating temperature?
200 Celsius
List in order of heat extraction rate transfer: air, water or oil quenching!
Air - 2.5-5 W/m2K
Oil - 20-500 W/m2K
Water - 100-15,000 W/m2K
What is the benefit of high Cr and C for cold worked tool steels?
Oxidation resistance due to Cr
What are characteristics of cold work tool steels?
Used for making tools for coke work applications, when the tool surface temperate does not rise more than 200 C.
•High abrasion, wear resistance, toughness and impact resistance
Also known as “non distorting steels”, as they show little change in dimension during heat treatment.
What are characteristics of oil/hardened steels?
The most widely used tool steel, hardened by oil quenching and contain high carbon with Mn, Cr and Mb.
•High as-quenched hardness
•High machinability
•Low red hardness
•High hardenability and they resist cracking on quenching
Cannot be used for cutting high speed or for hot working, as they will then soften.
Slow oil quenching can still form martensite, but causes less dimensional change, distortion and risk of cracking.
What are characteristics of air hardened steels?
They are suitable for applications where high toughness and fair wear resistance are required, e.g. blanking and forming dies.
Can be used in intricate shapes since the dimensional change after “quench” and temper is only ~0.25% of that in O1.
Principal alloying elements in addition to 1-2wt% C are Cr, Mn, Mo, V and Ni. Two important alloys are a2 and a4. They alloying additions make these alloys highly hardenable.
Fairly large sections may be air cooled to harden after austenitzing, larger sections need to be quenched to about 540 C and then can be air cooled from this temperature.
Large sections mean a greater temperature gradient between exterior and interior portions, crystal lattice change from outside to inside.
Austenitzing should be above 970 C, this will cause most of the C, Mo, Cr and V to dissolve in the austenite, however ~5% of residual carbides will remain undissolved.
Further heating would cause more to dissolve, but this is undesirable because it will result in the austenite grains growing and producing a very coarse grain structure.
What are applications of air hardened steels?
•Knives
•Blanking and trimming dies
•Coining dies
What are characteristics of high carbon, high chromium steels?
•Insufficient hardness at high speed and too brittle
•Excellent wear resistance and non-deforming properties (suitable for cold work die steels)
•High Cr content providing good oxidation resistance
•Usually air cooled to harden them
Can be hardened by oil or air hardening and contain 1.4-2.3%wt C and 12-14% Cr, with Mo, Co and V
What are applications for high carbon, high chromium steels?
•Mandrel for tube rolling by Pliger rolls
•Blanking and piercing dies, coining dies and drawing dies
