STAINLESS STEEL TUBE & PIPE   

Type 420 Stainless Steel provides both outstanding corrosion resistance and exceptional wear resistance. To obtain the best possible strength and wear resistance, oil quenching alloy 420 at a temperature between 1800° F and 1950° F. Alloy 420 Stainless Steel is commonly used in dental and surgical instruments, cutlery, plastics molds, pump shafts, steel balls, and numerous hand tool.

Because of its air hardening properties, type 420 Stainless Steel is not often welded, although, it is possible. When welded with stainless steel type 420, preheat the steel to a temperature between 300° F and 400° F. After welding, temper the work piece at temperature for two hours. When hot working, it is recommended to gradually raise the temperature to 1400° F and then gradually raise the temperature to anywhere from 2000° F to 2200° F. While working, avoid letting the work temperature drop below 1600° F by frequently reheating the steel. To prevent cracking, furnace cool the work piece slowly after working. Any extreme cold forming will cause alloy 420 Stainless Steel to crack, but it can endure minor cold work.

Grade 420 stainless steel is a higher carbon version of 410; like most non-stainless steels it can be hardened by heat treatment. It contains a minimum of 12 per cent chromium, just sufficient to give corrosion resistance properties. It has good ductility in the annealed condition but is capable of being hardened up to Rockwell Hardness 50HRC, the highest hardness of the 12 percent chromium grade. Best corrosion resistance is when the metal is hardened and surface ground or polished.

Related grades to 420 are high carbon high hardness martensitic stainless steel such as the 440 series and also variations to 420 containing molybdenum (for increased corrosion resistance), sulphur (for increased machinability) or vanadium (for higher hardness). A slightly higher carbon version of 420 is the non-standard grade 420C.

Martensitic stainless steel are optimised for high hardness, and other properties are to some degree compromised. Fabrication must be by methods that allow for poor weldability and usually also allow for a final harden and temper heat treatment. Corrosion resistance is lower than the common austenitic grades, and their useful operating temperature range is limited by their loss of ductility at sub-zero temperatures and loss of strength by over-tempering at elevated temperatures.

These properties are specified for bar product in ASTM A276. Similar but not necessarily identical properties are specified for other products such as stainless plate and forging in their respective specifications.

Typical compositional ranges for grade 420 stainless steel are given in table 1.

Table 1. Composition ranges for 420 grade stainless steel

Typical mechanical properties for grade 420 stainless steels are given in table 2.

Table 2. Mechanical properties of 420 grade stainless steel

Physical Properties

Typical physical properties for annealed grade 420 stainless steel are given in table 3.

Table 3. Physical properties of 420 grade stainless steel in the annealed condition

Grade Specification Comparison

Approximate grade comparisons for 420 stainless steels are given in table 4.

Table 4. Grade specifications for 420 grade stainless steel

Possible Alternative Grades

Possible alternative grades to grade 420 stainless steels are given in table 5.

Table 5. Possible alternative grades to 420 grade stainless steel

Corrosion Resistance

Grade 420 has good resistance in the hardened condition to the atmosphere, foods, fresh water and mild alkalies or acids. Corrosion resistance is lower in the annealed condition.

Performance is best with a smooth surface finish. Less corrosion resistant than the austenitic grades and also less than 17% chromium ferritic alloys such as Grade 430; 420 also has slightly lower resistance than grade 410.

This grade is commonly used for cutlery - particularly blades of table knives and for some carving knives and similar. The corrosion resistance is sufficient to resist attack by food and normal washing methods, but prolonged contact with unwashed food residues can result in pitting.

Heat Resistance

Not recommended for use in temperatures above the relevant tempering temperature, because of reduction in mechanical properties. The scaling temperature is approximately 650°C.

Heat Treatment

Annealing - Full anneal - 840-900°C, slow furnace cool to 600°C and then air cool.

Process Anneal - 735-785°C and air cool.

Hardening - Heat to 980-1035°C, followed by quenching in oil or air. Oil quenching is necessary for heavy sections. Temper at 150-370°C to obtain a wide variety of hardness values and mechanical properties as indicated in the accompanying table.

The tempering range 425-600°C should be avoided.

Welding

Pre-heat to 150-320°C and post-heat at 610-760°C. Grade 420 coated welding rods are recommended for high strength joints, where a post-weld hardening and tempering heat treatment is to be carried out.

If parts are to be used in the "as welded" condition, a ductile joint can be achieved by using Grade 309 filler rod. AS 1554.6 pre-qualifies welding of 420 with Grade 309 rods or electrodes.

Machining

In the annealed condition this grade is relatively easily machined, but if hardened to above 30HRC machining becomes more difficult. Free machining grade 416 is a very readily machined alternative.

Applications

Typical applications include:

•         Cutlery

•         Knife Blades

•         Surgical Instruments

•         Needle Valves

•         Shear Blades