Fracture-tough, high hardness stainless steel and method of making same

A stainless steel having improved fracture toughness and corrosion resistance at a given hardness level, consisting essentially of, in weight %, about 20 to about 30% Co, about 11 to about 13% Cr, about 5 to about 10% Ni, about 0.1 to about 0.5% Mo, about 0.2 to about 0.3% V, about 0.28 to about 0.32% C, and the balance iron, said steel having a cryogenically-formed martensitic microstructure tempered to include about 5 to about 10 volume % of post-deformation retained austenite dispersed therein and including M.sub.2 C-type carbides, where M is Cr, Mo, V, and/or Fe, dispersed therein.

A stainless steel having improved fracture toughness and corrosion resistance at a hardness level of at least about Rc 57, consisting essentially of, in weight %, about 21 to about 24% Co, about 11 to about 13% Cr, about 7 to about 9.50% Ni, about 0.1 to about 0.5% Mo, about 0.2 to about 0.3% V, about 0.28 to about 0.32% C, and the balance iron, said steel having a cryogenically-formed martensitic microstructure tempered to include about 5 to about 10 volume % of post-deformation retained austenite dispersed therein and including M.sub.2 C-type carbides, where M is Cr, Mo, V and/or Fe, dispersed therein.

A stainless steel having improved fracture toughness and corrosion resistance at a hardness level of at least Rc 60, consisting essentially of, in weight %, about 22.5% Co, about 12% Cr, about 8.50% Ni, about 0.3% Mo, about 0.25% V, about 0.30% C, and the balance iron, said steel having a cryogenically-formed martensitic microstructure tempered to include about 5 to about 10 volume % of post-deformation retained austenite dispersed therein and including M.sub.2 C-type oarbides, where M is Cr, Mo, V, and/or Fe, dispersed therein.

A stainless steel composition that is cryogenically-formable to produce a predominantly martensitic microstructure, consisting essentially of, in weight %, about 20 to about 30% Co, about 11 to about 13% Cr, about 5 to about 10% Ni, about 0.1 to about 0.5% Mo, about 0.2 to about 0.3% V, about 0.28 to about 0.32% C, and the balance iron.

A stainless steel composition that is cryogenically-formable to produce a predominantly martensitic microstructure consisting essentially of, in weight %, about 21 to about 24% Co, about 11 to about 13% Cr, about 7 to about 9.50% Ni, about 0.1 to about 0.5% Mo, about 0.2 to about 0.3% V, about 0.28 to about 0.32% C, and the balance iron.

A stainless steel composition that is cryogenically-formable to produce a predominantly martensitic microstructure consisting essentially of, in weight %, about 22.5% Co, about 12% Cr, about 8.50% Ni, about 0.3% Mo, about 0.25% V, about 0.30% C, and the balance iron.

A method of making a stainless steel having improved fracture toughness and corrosion resistance at a given hardness level, comprising the steps of:

• a) providing a stainless steel including at least about 11 weight % Cr for corrosion resistance, at least about 0.28 weight % C for temper hardness, a refractory metal carbide former in an amount selected to form M.sub.2 C-type carbides, where M is the refractory metal, Cr and/or Fe, Co and Ni in amounts selected to provide an as-quenched austenitic microstructure that is cryogenically-deformable to a martensitic microstructure including a minor amount of post deformation retained austenite dispersed therein, and the balance essentially Fe,
• b) cryogenically-deforming the steel in the as-quenched condition to transform the austenitic microstructure to a martensitic microstructure including a minor amount of post-deformation retained austenite dispersed therein, and
• c) tempering the cryogenically-deformed steel at an elevated temperature to control the amount of post-deformation retained austenite dispersed in the microstructure and to form the M.sub.2 C-type carbides dispersed in the microstructure.

A method of making a stainless steel having improved fracture toughness and corrosion resistance at a given hardness level, comprising the steps of:

• a) cryogenically deforming an as-quenched austenitic stainless steel consisting essentially of, in weight %, about 20 to about 30% Co, about 11 to about 13% Cr, about 5 to about 10% Ni, about 0.1 to about 0.5% Mo, about 0.2 to about 0.3% V, about 0.28 to about 0.32% C, and the balance iron, to form a martensitic microstructure including a minor amount of post-deformation retained austenite dispersed in the microstructure, and
• b) tempering the deformed stainless steel at an elevated temperature to provide about 5 to about 10 volume % of post-deformation retained austenite dispersed in the microstructure and to form M.sub.2 C-type carbides, where M is Cr, Fe, Mo and/or V, dispersed in the microstructure.

A method of making a stainless steel having improved fracture toughness and corrosion resistance at a hardness level of at least about Rc 57, comprising the steps of:

• a) cryogenically deforming an as-quenched austenitic stainless steel consisting essentially of, in weight %, about 21 to about 24% Co, about 11 to about 13% Cr, about 7 to about 9.50% Ni, about 0.1 to about 0.5% Mo, about 0.2 to about 0.3% V, about 0.28 to about 0.32% C, and the balance iron, to form a martensitic microstructure including less than about 15 volume % of post-deformation retained austenite dispersed in the microstructure, and
• b) tempering the deformed stainless steel at an elevated temperature to provide about 5 to about 10 volume % of post-deformation retained austenite dispersed in the microstructure and to form M.sub.2 C-type carbides, where M is Cr, Fe, Mo and/or V, dispersed in the microstructure.

A method of making a stainless steel having improved fracture toughness and corrosion resistance at a hardness level of at least about Rc 60, comprising the steps of:

• a) cryogenically deforming an as-quenched austenitic stainless steel consisting essentially of, in weight %, about 22.5% Co, about 12% Cr, about 8.50% Ni, about 0.3% Mo, about 0.25% V, about 0.30% C, and the balance iron, to form a martensitic microstructure including less than about 15 volume % of post-deformation retained austenite dispersed in the microstructure, and
• b) tempering the deformed stainless steel at an elevated temperature to provide about 5 to about 10 volume % of post-deformation retained austenite dispersed in the microstructure and to form M.sub.2 C. type carbides, where M is Cr, Fe, Mo and/or V, dispersed in the microstructure.

A cryogenically-formed and tempered stainless steel having improved fracture toughness and corrosion resistance, said steel including at least about 11 weight % Cr for corrosion resistance, C in an amount to achieve a hardness of at least about Rc 57, one or more refractory metal carbide formers in an amount selected to form M.sub.2 C-type carbides, where M is the refractory metal (s), Cr and/or Fe, Co and Ni in amounts selected to provide an as-quenched austenitic microstructure cryogenically-deformable to a martensitic microstructure including a minor amount of post deformation retained austenite, and the balance essentially Fe, said steel having a cryogenically-formed martensitic microstructure tempered to include a minor, controlled amount of post-deformation retained austenite and dispersed M.sub.2 C-type carbides.

A method of making a stainless steel having miproved fracture toughness and corrosion resistance, comprising the steps of:

• a) providing a stainless steel including at least about 11 weight % Cr for corrosion resistance, C in an amount to achieve a temper hardness of at least about Rc 57, a refractory metal carbide former in an amount selected to form M.sub.2 C-type carbides, where M is the refractory metal, Cr and/or Fe, Co and Ni in amounts selected to provide an as-quenched austenitic microstructure that is cryogenically-deformable to a martensitic microstructure including a minor amount of post deformation retained austenite dispersed therein, and the balance essentially Fe,
• b) cryogenically-deforming the steel in the as-quenched condition to transform the austenitic microstructure to a martensitic microstructure including a minor amount of post-deformation retained austenite dispersed therein, and
• c) tempering the cryogenically-deformed steel at an elevated temperature to control the amount of post-deformation retained austenite dispersed in the microstructure and to form the M.sub.2 C-type carbides dispersed in the microstructure, said tempered microstructure having a hardness of at least about Rc 57.