Cr50Ni50Nb versus NiCr28W 1

[0707]

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 ºC.

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

 

[Goahead]

If the supports are welded, welding also would be a concern. Why not taking a specimen of the new material, weld it if necessary, and testing it by immersing it in the actual fluid at the actual work cycles, for a few weeks to explore its behavior?

http://www.welding-advisers.com/

 

[0707]

The supports peaces will be cast in a mould, not welded

Thanks for sharing

Luis Marques

 

[stanweld]

I would suspect that the new material will have superior creep strength and resistance to thermal shock; however, you will be giving up considerable oxidation resistance.

 

[0707]

stanweld

I agree that creep resistance of NiCr28W will be slightly better above 950 ºC, but at temperatures lower than 700 ºC it will be more brittle than Cr50Ni50Nb, because NiCr28W as a high carbon content.

As refinery furnaces are cyclically submitted to slow downs and shut downs I doubt if NiCr28W would withstand these cyclic thermal shocks.

Let me know if I am thinking wrong.

Luis Marques

 

[EdStainless]

The 28W isn't stabilized. That might lead to some microstructure problems such as long term loss of ductility.
The W will give you better creep resistance.
Dropping clear to 28% Cr concerns me related to oxidation resistance.
I fail to see why this alloy would be less expensive. The Ni is the same, W isn't cheep, and Cr is only $2/lb.

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Plymouth Tube

 

[arunmrao]

0707,
I do not find any reason for cost savings between the two grades as the primary Ni content is same. Lower Cr might not result in significant cost savings. Also you mention that you are going to use cast pieces.

For castings, keeping the carbon low (0.1-0.15%) is always beneficial from casting properties and end use too. Higher carbon content of 0.3% might result in cracking,and early failure in service. Also if there is a welding operation,weld cracks form easily.

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[ijzer]

50/50Nb castings are generaly not selected for strength purpose but are selected for Heaters where the fuel contains large amounts of Na and or V. For selection see API 560.
Unless the fuel system of your furnace has been changed a change to 28/48/5W can be detremental.