Why is nickel added to steels?

[Donniev]

This is probably a foolish question, but I've read in a few different places tonight that the main reason for adding nickel in steel is to promote an austenitic structure, but since we want a martinsitic structure....? 

Or does it not matter because we're forcing the martensitic structure on the piece, and the added toughness that we get from nickel is why it's put in?

Sorry if it's a dumb question, I'm about 4 hours in tonight on reading what elements are added to steel and why, and how it effects (if at all) the HT. I'd like to make some hammers this spring and decided if I can't forge I'd get as much "book" knowledge as I can about different steels

[ThomasPowers]

Why do we want a martensitic structure?  Most smiths do ornamental work and want to avoid a martensitic structure like the plague!   Nickel promotes toughness and so a lot of armour plate alloys used it in the 20th century. Nickel also helps with corrosion resistance; many stainless steels include nickel as well as chromium 

 I mainly use nickel alloys for differentiation in pattern welded billets---not all of which are used for knives---(I've seen a pastry tool made of wrought iron and pure nickle---beautiful pattern and zilch carbon!)

[Frank Turley]

I recommend googling Bethlehem Steel's book, "Modern Steels and their Properties; Carbon and Alloy Steel Bars". I found the 4th edition dated 1958. Although dating from that period, I find the material quite informative. On pages 18-19, we find out that nickel provides a number of things when added to steel in appreciable amounts. It provides improved toughness particularly at lower temperatures; it provides a simplified and more economical heat treating procedures; it responds to milder quenching media; it lessens distortion in quenching; and it gives improved corrosion resistance (see Thomas' response above). It also lowers the critical temperatures of steel.

There is more regarding heat treatment internal structures.

[Donniev]

Thanks, I'll take a look at getting the book. I've found the more I dig looking for answers about certain alloys and the elements in them the more I want to know 

[patrick]

the primary reason to add nickel to the non-stainless steels is to increase their hardenability. Two bars of the same size but different compositon will have different hardenability. If the bars are large, say 10 inch diameter and one is 1045 and the other is a nickel bearing grade like 4340 you'll be able to make the 4340 much harder at the center than the 1045. This is important for industrial applications. 1045 has almost no nickel while 4340 has about 1.75%. Look up jominy testing and results for these two grades. That will help you understand what nickel does.