In our oil refinery, fired heaters internal convection bank supports which are not cooled are made of high alloy such as Cr50Ni50Nb. For new convection supports we are being invited to use NiCr28W, because it is cheaper and because the supports supplier says that NiCr28W offers better behaviour at high temperatures around 1150 %C2ºC. Chemical composition of Cr50Ni50Nb. Is as fallows: C = 0.1% max. Si = 1.0% max. Mn = 0.50% max. P = 0.020% max. S = 0.020% max. Cr = 48% to 52% Mo = 0.50% max. Ni = bal. Nb = 1.0% to 1.80% Fe = 1.0% max. N = 0.16% max. Chemical composition of NiCr28W Is as fallows: C = 0.35% to 0.55% Si = 1.0% to 2.0%. Mn = 1.50% max. P = 0.040% max. S = 0.030% max. Cr = 27% to 30% Mo = 0.50% max. Ni = 47.0% to 50.0%. Fe = bal. W = 4.0% to 6.0%. My concern is that the higher content of carbon of NiCr28W material turns this alloy less resistant to thermal shocks than Cr50Ni50Nb. Please share Best regards Luis Marques

Luis G-NiCr28W-(near equivalent to DIN 2.4879)- is a medium carbon heat resistant alloy suitable for use at higher temperatures of 1000 deg C +. Its creep resistance is better than 50Cr500NiNb grade - by 40%. Its ductility ( ductility will not drastically fall for this - as expressed by you) and strength (both mechanical and creep strength) is also higher at higher temperature of 950+ deg C to 1100 deg C. As centrifugally cast support this alloy will have 50% higher creep strength at 1100 deg C compared to 50Cr50NiNb grade or HP modified grade alloys. At 1100 deg C - if the intended service heater coil use - as given in your mail- then HK 40 (suggested by Krish) is unsuitable for such use. If used as a static support for heater coil -i.e., as a static cast grade support of 28CrNiW instead of centri-cast grade - you have to reduce the creep strength by 10-15% for 100,000 hr strength properties. This includes ductility and strength. Even then, this G CrNi28W will have more ductility, thermal shock resistance compared to 50Cr50NiNb or HK 40 or HP modified grade static cast alloy supports for heater service in your refinery. W addition of 4-5% in this G CrNi28 W - gives the extra strength, due to secondary hardening from W alloy additions- at higher temp of 950 deg C for this alloy. Corrosion resistance - due to fuel oil use in refinery - if V content in F.O, exceeds the 100 ppm limit - for static support of heater convection coils may not affect greatly. In my view, you should give due weight to the suggestion of your designer of support system for heater coil in choice of this alloy - except with 4-5% W in this alloy, any welding repair will give problem after prolonged use but not thermal shock resistance will be as good as HK 40 or HP modified for support system for heater coils. HK 40 or 50Cr50NiNb grades cannot stand the high temperature support strength at 950 deg C+ to 1100 deg C if the support system will see a continuous temp of 950 deg C+ in your heater coil convection zone. Hope this helps you for your choice C.V.Srinivasan Nishi Engineers Pvt Ltd India Feb 23, 2008 E-mail: nishi@vsnl.com >In our oil refinery, fired heaters internal convection bank >supports which are not cooled are made of high alloy such as >Cr50Ni50Nb. > >For new convection supports we are being invited to use >NiCr28W, because it is cheaper and because the supports >supplier says that NiCr28W offers better behaviour at high >temperatures around 1150 %C2ºC. > >Chemical composition of Cr50Ni50Nb. >Is as fallows: C = 0.1% max. Si = 1.0% max. Mn = 0.50% max. >P = 0.020% max. S = 0.020% max. Cr = 48% to 52% Mo = 0.50% >max. Ni = bal. Nb = 1.0% to 1.80% Fe = 1.0% max. N = 0.16% >max. > >Chemical composition of NiCr28W >Is as fallows: C = 0.35% to 0.55% Si = 1.0% to 2.0%. Mn = >1.50% max. P = 0.040% max. S = 0.030% max. Cr = 27% to 30% >Mo = 0.50% max. Ni = 47.0% to 50.0%. Fe = bal. W = 4.0% to >6.0%. > >My concern is that the higher content of carbon of NiCr28W >material turns this alloy less resistant to thermal shocks >than Cr50Ni50Nb. > >Please share > >Best regards > >Luis Marques