Ferritic and precipitation hardening stainless steels have different compositions and properties. Ferritic stainless steels have chromium contents close to or less than SS 304, and are divided into groups based on their chromium levels and alloying elements. They have good corrosion resistance but lower strength compared to austenitic grades. Precipitation hardening stainless steels are strengthened through heat treatment rather than relying on martensite, and are divided into martensitic, semi-austenitic, and austenitic types based on their compositions. They have strengths up to three times that of austenitic grades through controlled precipitation during heat treatment, while maintaining good corrosion resistance and weldability.
5. Group 1 (Type 409/410L)
Grade 409 is favourable in moist conditions and is less prone to
corrosive cracking. Primarily it was used in exhaust systems of
automobiles but now it is being used in exhaust tubing and
catalytic convertors casings. On the other hand, grade 410 is used
in making outer bodies of buses, containers and LCD monitor
frames, etc.
• Type 409—cheapest type; used for automobile exhausts; ferritic
(iron/chromium only).
• Type 410—martensitic (high-strength iron/chromium). Wear-
resistant, but less corrosion-resistant. C~0.15
• Type 410 L—C ~0.03
6. Group 2 (Type 430)
• Grade 430 is more resistant to corrosive attacks by organic,
nitric and sulphuric acids as it has a high chromium content.
• The major areas of its application is in the manufacturing of
interior walls of washing machines, kitchen sinks, cooking
utensils and indoor panels etc.
7. Group 3 (Type 430 Ti, 439 and 441)
• However, sometimes SSs get prone to sensitization in the WAZ
(weld affected zone). Nowadays, they are being used as a
substitute for austenitic grade 304 in the manufacturing of
exhaust manifold systems, interior walls/ sheets of various
stainless steel’s appliances.
• More recently developed ferritic grades, such as 439 and 441 meet an
even wider range of requirements. They can be formed into more
complex shapes and joined using most conventional joining methods,
including welding. In material selection decisions, these grades are
often weighed against 304 austenitic grades.
8. Molybdenum in Ferritic SSs
• The addition of molybdenum enhances the resistance of ferritic
stainless steels to localised corrosion (434, 436). Grade 444 is even
considered at least equal to austenitic grade 316 in most of the cases
when considering corrosion resistance properties.
9. Ni and Mo in Ferritic SSs
• Superferritic grades have also been developed since many years. Their
very high chromium content (25-29%) with additional Ni and Mo
alloying make them well-known highly corrosion resistant products
albeit restricted to marginal applications. This is due to their high
sensitivity to embrittling phase transitions.
13. Precipitation Hardening Stainless steels
• The family of precipitation hardening stainless steels is hardened by
precipitation instead of relying upon martensitic strength
• These can be divided into three main types
• low carbon martensitic
• semi-austenitic
• austenitic
14. Alloying
• Precipitation hardening is achieved by the addition of copper, molybdenum,
aluminium and titanium either singly or in combination
15. Type C % Mn % Si % Cr % Ni % Mo % Others %
17-4 PH 0.04 0.40 0.50 16.50 4.25 -
0.25 Cb,
3.60 Cu
17-7 PH 0.07 0.70 0.40 17.00 7.00 - 1.15 Al
PH 15-7
Mo
0.07 0.70 0.40 15.00 7.00 2.25 1.15 Al
Precipitation Hardening Steels
16. Mechanical behaviour
• The precipitation hardening (PH) stainless steels are a family of corrosion
resistant alloys some of which can be heat treated to provide tensile strengths
of 850MPa to 1700MPa and yield strengths of 520MPA to over 1500MPa -
some three or four times that of an austenitic stainless steel such as type 304
or type 316.
• They are used in the oil and gas, nuclear and aerospace industries where a
combination of high strength, corrosion resistance and a generally low but
acceptable degree of toughness is required.
17. Good weldability
They have good weldability and their corrosion
resistance is comparable to standard austenitic
steels grade like 304.
