Alloy 420 is a hardenable, martensitic stainless steel that is a modification of Alloy 410. Similar to 410, it contains a minimum of 12% chromium, just sufficient enough to give corrosion resistant properties. Alloy 420 has higher carbon content than Alloy 410 which is designed to optimize strength and hardness characteristics. It has good ductility it the annealed condition but is capable of being hardened to a minimum Rockwell hardness of 50 HRC, the highest hardness of the 12% chromium grades. Due to its hardening properties, Alloy 420 is not often welded, although it is possible. Martensitic stainless steels are designed for high hardness and sometimes other properties are to some degree compromised. Corrosion resistance is lower than the common austenitic grades and their useful operating range is limited by their loss of ductility at sub-zero temperatures and loss of strength by over-tempering at elevated temperatures. Its best corrosion resistance is achieved when the metal is hardened and surface ground or polished.

Features

CORROSION RESISTANCE:

l  Less resistant than the austenitic grades and the 17% chromium ferritic alloys

l  Good resistance in the hardened condition to the atmosphere, foods, fresh water, and mild acids

l  Resistance lowered in the annealed condition

l  Best with a smooth surface finish

HEAT RESISTANCE:

l  Not recommended for use above the relevant tempering temperature because of reduction in mechanical properties

l  Scaling temperature is approximately 1202°F (650°C)

MACHINABILITY

·         Tough, stringy chip build-up.

·         Similar to machining some of the high carbon tool steel

WELDING

·         Not commonly welded due to air hardening characteristics.

·         Welding may be performed after preheating to 300-400 F

·         Post weld tempering at temperature for 2 hours

HOT WORKING

·         Recommended to bring temperatures slowly up to 1400, then on to 2000-2200 F

·         Furnace cool slowly to avoid cracking, after furnace working

·         Reheating is necessary to keep working temperature above 1600 F.

COLD WORKING

·         Can withstand only minor cold work.

·         Radical forming operations will result in cracking.

ANNEALING

·         Anneal at temperatures between 1550-1650 F (843-900 C)

·         Allow for slow furnace cooling.

TEMPERING

·         Temper at temperatures of 300-400 F

·         Air cool for maximum hardness and corrosion resistance.

HARDENING

·         Soak at 1850-1950 F to quench in heated oil.

Application

Alloy 420 is used for a variety of applications where good corrosion and outstanding hardness is necessary. It is not usually used at temperatures exceeding 800oF (427oC) due to quick hardening and loss of corrosion resistance. Examples of applications that use alloy 420 include:

·         Cuttery

·         Knife blades

·         Surgical instruments

·         Needle valves

·         Shear blades

·         Scissors

·         Hand tools