Method of heat treatment of medium- and high-carbon alloy steel and use thereof

Abstract

Exemplary arrangements are method for heat treatment of medium and high carbon alloy steels. The exemplary methods are operative to produce a fragmented multiphase microstructure consisting of tempered bainitic ferrite and martensite, separated by layers of stable residual austenite and finally dispersed alloy carbide precipitates which are operative to produce secondary hardness. The exemplary methods make it possible to control the phase fraction and morphology of the microstructure enabling achievement of desired properties of the steel including improved impact strength and resistance to fracture.

1 - 10 . (canceled) 
     
     
         11 . A method of heat treating an article comprised of medium to high carbon alloy steel configured to undergo a bainitic transformation, comprising:
 a) heating the article to an austenitizing temperature T A ,   b) subsequent to (a), maintaining the article at the austenitizing temperature T A  for a time t A ,   c) subsequent to (b), cooling the article to an isothermal annealing temperature T B ,   wherein 350° C.≥T B >M s (γ)+10° C., where M s (γ) is the martensitic transformation start temperature,   d) subsequent to (c), holding the article at the isothermal annealing temperature T B  for a time t B  sufficient to produce within the article a volume fraction of 10% to 70% bainitic ferrite,   e) subsequent to (d), cooling the article to a temperature T Q , wherein T Q  has a value between a matensitic transformation start M s (γ R ) and a martensitic transformation finish M f (γ R ) of residual austenite remaining in the article after (d),   f) subsequent to (e), holding the article at the temperature T Q  for a time t Q  until temperature is equalized throughout the entire article,   subsequent to (f), at least one of
 g1) tempering the article by heating and cooling the article at least once by heating the article to a tempering temperature T T  for a tempering time t T  to produce a secondary hardness effect by precipitation strengthening by alloy carbides, or 
 g2) carbon partitioning the article by heating the article to a partitioning temperature T P  and then holding the article at the partitioning temperature T P  for a partitioning temperature time t p  sufficient to defuse martensite and bainitic ferrite to austenite to increase thermal and mechanical stability. 
   
     
     
         12 . The method according to  claim 11   wherein g1) is carried out, and   h) concurrently with at least a portion of g1, treating a surface of the article.   
     
     
         13 . The method according to  claim 11   wherein g1 is carried out, and   h) concurrently with at least a portion of g1, treating a surface of the article by nitriding or physical vapor deposition.   
     
     
         14 . The method according to  claim 11   wherein g1 is carried out, and includes initially heating the article wherein T T  is between 400° C. to 600° C. and t T  is between one to three hours.   
     
     
         15 . The method according to  claim 11   wherein g1 is carried out, and wherein during cooling a martensitic transformation occurs, and   h) subsequent to g1, further tempering by heating the article to a temperature of less than or equal to 400° C. to reduce embrittlement of the martensite produced in g1.   
     
     
         16 . The method according to  claim 11   wherein in (a) and (b) T A  corresponds to a martensitic transformation start temperature M s (γ) of less than or equal to 340° C.   
     
     
         17 . The method according to  claim 11   wherein in (c)   the article is cooled either via continuous cooling or cooling with at least one intermediate isothermal stop, and   wherein T B  is below the temperature of bainitic transformation start (B s ) of the steel and at least 10° C. above M s (γ), and wherein T B  is greater than or equal to 200° C.   
     
     
         18 . The method according to  claim 11   wherein g1 is carried out, and   wherein subsequent to g1 there is at least 10% by volume of the residual austenite in the final microstructure of the steel.   
     
     
         19 . The method according to  claim 11   wherein in (a) through (g) the steel includes alloying additives (in percent by weight) of:   0.4-2.3% C, 0.4-2.5% Mn, 0.5-3.0% Si, at least one of the following carbide-forming elements: Cr up to 17.0%, Mo up to 10.0%, V up to 4.0% while maintaining the relation: (% Si+% Al)≥0.8% and optional additives from the group: W up to 18.0%, Ti up to 0.2%, Nb up to 0 1%, Al up to 2.0%, Ni up to 4.5%, Co up to 10.0%, Cu up to 1.2%, the remaining components are iron and trace amounts of unavoidable admixtures.   
     
     
         20 . The method according to  claim 11   wherein in (a) T A  is about 1050° C.   
     
     
         21 . The method according to  claim 11   wherein in (b) t A  is about 15 minutes.   
     
     
         22 . The method according to  claim 11   wherein step g1 is carried out a plurality of times,   wherein the respective tempering temperature is higher in a first step g1 then in a subsequent step g1.   
     
     
         23 . The method according to  claim 11   wherein in (c) the isothermal annealing temperature T B  is about 25° C. above the martensitic transformation start temperature.   
     
     
         24 . A method of treating a steel article comprised of medium to high carbon alloy steel configured to undergo a bainitic transformation comprising:
 a) heating the steel article to an austenitizing temperature T A  that is greater than a critical temperature for austenitizing, and holding the steel article at about T A  for about an austenitizing time t A ,   b) subsequent to (a), cooling the steel article to an isothermal annealing temperature T B  wherein the isothermal annealing temperature is
 less than or equal to 350° C., and 
 greater than the martensitic transformation start temperature of the steel+10° C., 
   and then holding the steel article at the isothermal annealing temperature T B  for a time t b  until 10% to 70% of the volume fraction of bainitic ferrite is produced,   c) subsequent to (b) cooling the steel article to a quench temperature T Q , wherein the quench temperature has a value between a martensitic transformation start temperature M s (γ r ) and a martensitic transformation finish temperature M f (γ r ) of residual austenite which remains after the step (b), and   then holding the steel at the quench temperature T Q  for a quench holding time t Q  sufficient to equalize the temperature in the steel article,   d) tempering the steel article at least one time by heating the steel article to a tempering temperature T T  and holding the article at such tempering temperature for a tempering time t T  to produce a secondary hardness effect in the treated steel article until precipitation strengthening by alloy carbides is achieved.   
     
     
         25 . The method according to  claim 24  wherein in (d) in a first tempering time the tempering temperature T T  is in the range of 400° C. to 600° C. and the tempering time t T  is in the range of one hour to three hours. 
     
     
         26 . The method according to  claim 24   wherein subsequent to (d) in cooling after tempering a martensitic transformation occurs,   and further comprising:   subsequent to (d), carrying out a further tempering step in which the tempering temperature T T  is less than 400° C. wherein embrittlement of newly formed martensite is reduced.   
     
     
         27 . The method according to  claim 24   wherein in (a) at the austenitizing temperature T A  and austenitizing time t A  a martensitic transformation start temperature M s (γ) less than or equal to 340° C. is obtained.   
     
     
         28 . The method according to  claim 24   wherein in (b) partial bainitization is carried out through one or more isothermal stops, or by continuous cooling at an isothermal annealing temperature T B  of at least 350° C. but less than the martensitic transformation start temperature M s (γ)+10° C., and   wherein the isothermal annealing temperature T B  is
 below the temperature of a bainitic transformation start, 
 at least 10° C. above the martensitic transformation start temperature, and 
 least 200° C. 
   
     
     
         29 . The method according to  claim 24   wherein upon completion of (d) there is at least 10% by volume of the residual austenite in the final microstructure of the steel article.   
     
     
         30 . The method according to  claim 24  and further comprising:
 concurrently with at least a portion of (d), treating a surface of the article by nitriding or physical vapor deposition.