Fabrication of 18% Ni maraging steel laminates by roll bonding

A method of fabricating 18% Ni maraging steel laminates of varying alloy compositions by roll bonding. Laminates composed of a plurality of different grades of Cobalt-containing or Cobalt-free maraging steels are disclosed.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The invention relates to the fabrication of 18% Ni maraging steel laminates 
of varying alloy compositions by roll bonding. 
2. Description of the Prior Art 
Maraging steels are iron-nickel martensites that are hardened by the 
precipitation of Mo and Ti containing intermetallic compounds. Cobalt, 
molybdenum, and titanium are added in various amounts to the basic alloy 
to produce a final product of varying strength levels from 200 to 350 KSI. 
Methods of manufacturing cobalt-containing maraging steels of a specific 
alloy formulation are well known in the prior art. Recently, U.S. Pat. No. 
4,443,254 describes a method for producing a cobalt-free version of the 
250 and 300 KSI strength levels of the 18% Ni maraging steels. 
It is known that steel can be roll bonded into laminate form. The steels to 
be bonded are heated to a selected temperature and then pressed together 
under high pressure to effect the bond. However, in quenched and tempered 
steels, there is a strong tendency for the laminates to separate and 
buckle during the quenching process due to non-uniform dimensional changes 
induced by rapid cooling and non-uniform expansion due to the formation of 
martensite. 
The present invention addresses fabrication and lamination of a 
precipitation hardening type steel which can be air cooled from both the 
solution and aging temperature. Thus, the difficulties associated with the 
rapid cooling rate in the heat treatment of the quenched and tempered 
steel is avoided in the slow cooling of the maraging steels. 
SUMMARY OF THE INVENTION 
The invention may be summarized as a method of fabricating selected 
combinations of 18% Ni maraging steels to produce a material of enhanced 
properties, for example, increased ballistic performance. Hardening, which 
is achieved in the maraging steels during aging, is retained at room 
temperature by air cooling. A rapid quench, such as water or oil, is not 
required. The various grades (strength/hardness levels) are all compatible 
to the same heat treatment and have very similar coefficients of thermal 
expansion (5.0 to 7.5.times.10.sup.-6 /.degree.F. from 70.degree. to 
900.degree. F.). 
Laminates composed of a plurality of different grades of cobalt-containing 
(C) or cobalt-free (T) maraging steels may be tailored to provide 
solutions to problems not otherwise solvable by the use of a single grade 
of material. For instance, a high hardness grade can be used as a front 
material and a lower hardness grade for the rear material to enhance 
ballistic performance. The front harder grade will serve to break up the 
projectile and the lower hardness rear material will act in an impact 
absorbing role. 
In addition to avoiding the problems of warping and delamination which 
result from the rapid cooling of quenched and tempered steels, maraging 
steel laminates can be expected to possess the known improved formability 
and weldability characteristics of their component layers. The maraging 
steel laminates will also be more cost effective to produce due to their 
inherently less expensive air cooling process and a higher yield of 
product without defects. 
The method of the invention comprises the steps of providing a quantity of 
at least two different alloys of maraging steel, forming sheets of each of 
a selected thickness, roll bonding the sheets together at an elevated 
temperature, and cooling the resulting laminate in air to ambient 
temperature. 
The resulting laminate is a unique material not heretofore available having 
the properties and advantages described above.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
In accordance with the general method of the disclosure, specific laminate 
combinations have been fabricated by the following procedure. 
Quantities of maraging steel charge stock are formed into twenty inch 
ingots by a series of melting steps comprising argon-oxygen decarburizing, 
vacuum induction melting, and finally vacuum arc melting. The ingots are 
heated to a homogenization temperature (2200.degree.-2275.degree. F.) and 
upset forged into fourteen by six inch length slabs which are then 
conditioned (ground to remove surface defects), heated to a temperature of 
1950.degree.-2000.degree. F. and rolled into 0.10-0.50 inch thickness 
plates. The plates are ground, cleaned, sized, and matched with a desired 
laminate mate. The selected combinations are then heated to a temperature 
of 1900.degree.-2000.degree. F. and rerolled to effect the bond. The 
resulting laminates are air cooled to ambient temperature and mechanically 
finished to size. 
Maraging steels of particular interest which were chosen for initial 
experimentation are cobalt-containing (C) and cobalt-free (T) alloys 
available from Teledyne Vasco, Latrobe, PA 15650. These are designated by 
their alloy component and KSI strength level. For cobalt-containing alloys 
there are C-200, C-250, C-300, and C-350 stock available. For cobalt-free, 
T-200, T-250, and T-300. 
The nominal compositions of the cobalt-containing and cobalt-free alloys 
are as follows: 
______________________________________ 
Alloy Ni % Co % Mo % Ti % Al % Fe % 
______________________________________ 
C-200 18.5 8.5 3.25 .20 .10 Balance 
C-250 18.5 7.5 4.80 .40 .10 " 
C-300 18.5 9.0 4.80 .60 .10 " 
C-350 18.5 12.0 4.80 1.40 .10 " 
T-200 18.5 0 3.00 .70 .10 " 
T-250 18.5 0 3.00 1.40 .10 " 
T-300 18.5 0 4.00 1.85 .10 " 
______________________________________ 
The balance of the compositions of the cobalt-containing and cobalt-free 
alloys shown in the Fe % column, while being substantially iron, also 
includes small amounts of silicon (0.10% max.), manganese (0.10% max.), 
carbon (0.03 max.), sulfur (0.01% max.) and phosphorus (0.01% max.). The 
balance of the cobalt-containing alloys additionally includes small 
amounts of zirconium (0.01%) and boron (0.003%). 
Laminate combinations which were fabricated by the above described method 
include 
T-300--T-250 
T-300--T-200 
C-350--T-250 
C-350--T-200 
Having fully described the method of preparing and product comprising a 
novel maraging steel laminate, the invention is hereby defined by the 
following claims.