Evolution of Boiler tube materials

 Power Plant heat rate is directly related to  Boiler technology and its metallurgy. Over the years of research and development boiler materials are improved to withstand hi temperature stress and corrosion environment. Basically it has divided into four generation of steel depending upon the creep strength.

We can see in the below curve development of steel w.r.t creep strength from the year of 1920 to 2000

Generation	Years	Alloy Modifications	Strength 105 hr Creep Rupture Achieved kg/cm2	Example Alloys	Maximum Metal Use Temp. °C
1	1960-70	Addition of Mo or Nb, V to simple 12Cr and 9Cr Mo steels	611.82	EM 12, HCM9M, HT9, Tempaloy F9.HT91	565
2	1970-85	Optimization of C, Nb.V	1019.71	HCMI2.T91, HCM2S	593
3	1985-95	Partial substitution of W Tor Mo	1427.60	P-92, P-122 (HCMI2A, NF616)	620
4	Emerging	Increase of W and addition of Co	1835.48	NFI2.SAVEI2	650

Typical Boiler tube materials are used depending on the temperature zone and corrosion environment of  different area of the Boiler. Below fig shows boiler pressure parts materials as per temperature zone:

It is observed that percentage of Cr is the main factor to withstand the hi temperature stress. Below fig shows that specially in super heater region hi grade of steel is being used which has hi percentage of Cr .

Typical Tubing for Super Critical Boiler