Process for manufacturing semi-finished products from sintered refractory metal alloys

In a process for manufacturing semi-finished products from sintered refractory metal alloys with a stacked microstructure, the sinter feed reshaped by at least 85% is subjected prior to recrystallization annealing to an intermediate annealing for at least 20 minutes at a temperature not less than 700.degree. C. and not greater than that at which no further recrystallization occurs. Following this intermediate annealing, the hot feed is deformed by a further 3% to 30%. The process makes it possible to manufacture semi-finished products with a good stacked microstructure and of substantially greater dimensions, or of the same dimensions and a substantially better stacked microstructure, than can be obtained with known manufacturing processes.

The invention covers a process for manufacturing semi-finished products 
from sintered refractory metal alloys having a stacked microstructure, in 
which the sintered product is reshaped by at least 85% by mechanical 
deformation in several reshaping steps and is then subjected to a 
recrystallization annealing treatment. 
In order to improve the hot strength and creeping strength of refractory 
metals at high temperatures, various methods of alloying refractory metals 
have so far been developed. 
According to a known process limited to powder metallurgy, a refractory 
base metal is treated with certain elements and is subjected to intensive 
mechanical reshaping during manufacture, achieving a reshaping factor of 
at least 85%. In this manner, the refractory metal alloy assumes a very 
specific type of microstructure, the so-called stacked microstructure, 
which is characterized by elongated granules with a length/width ratio of 
at least 2 to 1. 
Examples of known refractory metal alloys of this type are tungsten and 
molybdenum alloys treated with small amounts of aluminum, silicon and 
potassium, or with silicon and potassium. 
To manufacture these alloys, the sintered base material is heated to a 
temperature of between about 1350.degree. C. and about 1450.degree. C. and 
is then reshaped by mechanical deformation, such as by rolling or 
round-forging and drawing, in several stages up to a final reshaping 
factor of 85%. The reshaping factor is a measure for the degree of plastic 
deformation that has been achieved and can be calculated by the formula 
##EQU1## 
where A.sub.a stands for the cross-sectional area of the sintered base 
material and A.sub.e stands for the cross-sectional area of the finished 
product. To facilitate reshaping and to avoid cracks in the material, it 
is important to maintain the required reshaping temperature during the 
entire reshaping process, so that reheating is usually necessary between 
the various reshaping stages. Following completion of the reshaping 
process, the material is subjected to a recrystallization annealing 
treatment. The recrystallization temperature depends on the type of alloy 
and on the degree of reshaping that has been applied. The higher the 
degree of reshaping, the higher will be the temperature required for 
recrystallization with this type of alloy. 
It is a disadvantage of this process for manufacturing refractory metal 
alloys with a stacked microstructure that the process is limited to 
semi-finished products of relatively small dimensions, e.g. a maximum 
thickness of about 2 mm for sheet and a maximum diameter of about 1.7 mm 
for wire. A satisfactory stacked microstructure can as a rule not be 
achieved for semi-finished products exceeding these dimensions. 
Special molybdenum alloys with a stacked microstructure are described in EU 
A1 119 436 in which the molybdenum is treated with approximately 0.005% to 
0.75% by weight of at least one of the elements aluminum, silicon and 
potassium. This pre-publication also states that the high-temperature 
properties of the alloy can be further improved by treating this alloy 
with 0.2% to 3% by weight of at least one compound selected from the group 
of oxides, carbides, borides and nitrides of the elements La, Ce, Dy, I, 
In, Ti, Zr, Nb, Ta, Hf, V, Cr, Mo, W and Mg. 
In manufacturing these special molybdenum alloys the sintered base material 
is reshaped with a reshaping factor of at least 85%, but preferably 95% 
and higher. As a particularly advantageous step, a first recrystallization 
annealing treatment is recommended after achieving a reshaping factor of 
between 45% and 85%. Subsequently, the material is reshaped further until 
the intended reshaping factor is reached, followed by a final 
recrystallization annealing step. No special directions are given 
concerning successive reshaping factors when reshaping the material 
further up to the desired reshaping factor. This special manufacturing 
process results in a certain improvement in the creeping strength and the 
high-temperature properties of these alloys compared to alloys that are 
manufactured without the intermediate subcritical annealing step. However, 
this manufacturing process also does not permit the production of 
semi-finished products from molybdenum alloys with a stacked 
microstructure whose sheet thickness or wire diameter are of greater 
dimension than those stated at the beginning. 
It is the objective of the invention at hand to create a process for 
manufacturing semi-finished products of sintered refractory metal alloys 
with a stacked microstructure, suitable for producing semi-finished 
products of relatively large dimensions, or for achieving a substantially 
improved stacked microstructure as compared to the state of technology as 
described when producing semi-finished products having the same dimensions 
as heretofore. 
In accordance with the invention, this is achieved by subjecting the sinter 
feed material, having been reshaped by at least 85%, to an intermediate 
annealing treatment during at least 20 minutes at a minimum temperature of 
700.degree. C. and a maximum temperature just short of recrystallization, 
and subsequently reshaping the material in a heated condition by an 
additional 3% to 30%. 
The combination of the special intermediate annealing step for a base 
material that has been reshaped by at least 85%, with a subsequent 
reshaping step within a very specific range of reshaping, yields the 
completely surprising result that semi-finished products of sintered 
refractory metal alloys with a good stacked microstructure can be produced 
with substantially greater dimensions compared to semi-finished products 
manufactured by known processes, or which have a much improved stacked 
microstructure compared to the state of technology as described. 
In this manner, the process according to the invention permits the 
production of sheet with thicknesses of up to about 10 mm and of rods 
having a diameter of up to about 50 mm, while at the same time forming a 
satisfactory stacked microstructure. 
The intermediate annealing step and subsequent reshaping step can be 
repeated once or several times, with repetitions possible prior to as well 
as following the/a first recrystallization annealing step. The only 
absolute requirement is that the first intermediate annealing treatment 
and the subsequent reshaping step must occur prior to a first 
recrystallization annealing treatment. It is also important that 
intermediate annealing treatments and reshaping steps should only be 
carried out in combination with each other as long as the material has not 
been subjected to a first recrystallization annealing step. 
Additional recrystallization annealing steps following a repeat cycle of 
intermediate annealing steps and reshaping steps can result in an 
additional improvement in the stacked microstructure compared to material 
that has been subjected to only one subcritical annealing step. 
In case of a repeat cycle, the additional reshaping steps of from 3% to 30% 
relate to the respective cross-section of the material during the 
preceding annealing step. 
The process according to the invention is particularly suited to refractory 
metal alloys of molybdenum, tungsten, chromium and to alloys of 
combinations of these metals, which have been treated with aluminum, 
potassium and silicon or with compounds and/or mixed phases from the group 
of oxides, nitrides, carbides, borides, silicates or aluminates having a 
melting point in excess of 1500.degree. C.

The manufacturing process according to the invention will now be described 
in more detail by way of examples. 
EXAMPLE 1 
Potassium silicate solutions were sprayed into molybdenum oxide, which was 
then reduced to MoO.sub.2 in a first step at approximately 650.degree. C. 
in an H.sub.2 counterflow, and further to molybdenum metal powder in a 
second step at approximately 1100.degree. C. The amount sprayed in was 
apportioned so that the metal powder contained 0.175% by weight of silicon 
and 0.152% by weight of potassium. 
The molybdenum powder with an average grain size of about 5 .mu.m was then 
pressed into plates of size 550 mm.times.200 mm.times.70 mm on a die press 
at 3 MN. 
Subsequently, the plates were sintered under an inert H.sub.2 gas cover, 
using a heating time of 3 hours and a hold time of 5 hours at 1000.degree. 
C. 
The sintered plates were rolled, beginning at a reshaping temperature of 
about 1400.degree. C., into sheets of 5.6 mm thickness in steps of 
approximately 10% reshaping at a time. After annealing at 1100.degree. C. 
under an inert H.sub.2 cover during 5 hours, the sheet was rolled down to 
the final 5 mm thickness. 
Following a final recrystallization annealing step at 1900.degree. C. 
during 15 minutes, the sheet assumed a stacked microstructure. The creep 
rate of this sheet amounted to 
##EQU2## 
at 1800.degree. C. and a load of 10N/mm.sup.2. 
It is also feasible to roll the 5 mm sheet down to 4.5 mm in one step after 
recrystallization annealing. In this case, the additional intermediate 
annealing step at 1100.degree. C. and the additional final 
recrystallization annealing treatment can be omitted. 
EXAMPLE 2 
98.8% by weight of molybdenum powder with a mean grain size of about 5 
.mu.m was blended with 1.2% by weight of La(OH).sub.3 powder with a mean 
grain size of 0.4 .mu.m in a mixing unit and was then pressed into plates 
of size 170 mm.times.400 mm.times.54 mm on a die press at 3 MN. 
Subsequently, the plates were sintered under an inert H.sub.2 gas cover, 
using a heating time of 3 hours and a hold time of 4 hours at 2000.degree. 
C. 
The sintered plates were rolled, beginning at a reshaping temperature of 
about 1400.degree. C., into sheets of 2.2 mm thickness in steps of 
approximately 10% reshaping at a time. After annealing at 1100.degree. C. 
under an inert H.sub.2 cover during 5 hours, the sheet was rolled down to 
the final 2 mm thickness. 
Following a final recrystallization annealing step at 2300.degree. C. 
during 15 minutes, the sheet assumed a stacked microstructure, with the 
grains showing an average length/width ratio of 5:1. The creep rate of 
this sheet amounted to 
##EQU3## 
at 1800.degree. C. and a load of 10N/mm.sup.2. 
EXAMPLE 3 
95.3% by weight of molybdenum powder with a mean grain size of about 5 
.mu.m was combined with 4.7% by weight of La(OH).sub.3 powder with a mean 
grain size of 0.4 .mu.m and made into sheet of 2 mm thickness under the 
same conditions as in Example 2. 
The final recrystallization annealing step took place at 2300.degree. C. 
during 15 minutes. The resulting stacked microstructure showed grains with 
an average length/width ratio exceeding 10:1. 
EXAMPLE 4 
Blue tungsten oxide powder was blended with solutions of potassium silicate 
and aluminum chloride and reduced to a treated metal powder with a mean 
grain size of about 5 .mu.m under an inert H.sub.2 gas cover, containing 
0.16% by weight of potassium, 0.19% by weight of silicon and 0.027% by 
weight of aluminum. 
The powder was washed with hydrofluoric acid and pressed isostatically into 
square rods with a cross-section of 2 cm.times.2 cm at a pressure of 3 MN. 
Following a heating time of 5 hours, the rods were sintered under an inert 
H.sub.2 cover at 2600.degree. C. for 5 hours. Starting at reshaping 
temperatures of 1600.degree. C., the sintered rods were forged into rods 
of 7 mm diameter in reshaping steps of about 10% each and were then drawn 
into wire with a diameter of 5.15 mm. After annealing under an inert 
H.sub.2 cover at 1250.degree. C. for 3 hours, the wire was drawn down 
further to a diameter of 5 mm in a single step. 
The stacked microstructure was formed during a 15-minute recrystallization 
annealing treatment at 2300.degree. C. 
EXAMPLE 5 
Molybdenum oxide powder was treated with a potassium silicate solution in 
such a manner that, after reduction, a mixture of molybdenum with 0.2% by 
weight of potassium and 0.315% by weight of silicon was obtained. This 
treated molybdenum powder was blended with an equal quantity of chromium 
powder and pressed into plates measuring 400 mm.times.170 mm.times.40 mm 
on a die press at a pressure of 3 MN. 
The plates were then sintered under an inert H.sub.2 gas cover, using a 
heating time of 3 hours and a hold time of 7 hours at 1700.degree. C. The 
sintered plates were rolled, beginning at a reshaping temperature of about 
1200.degree. C., into sheets of 3.3 mm thickness in steps of approximately 
10% reshaping at a time. 
After annealing in a vacuum at 880.degree. C. during 5 hours, the sheet was 
rolled down to the final 2 mm thickness at a temperature of 700.degree. C. 
The stacked microstructure was formed during a final 15-minute 
recrystallization annealing treatment at 1700.degree. C. 
EXAMPLE 6 
This example compares the manufacture of semi-finished products of equal 
dimensions, on the one hand based on established technology and on the 
other hand according to the process covered by the invention. 
It can be seen that the creep rate of the semi-finished product made by the 
process according to the invention is much lower and thus has a stacked 
microstructure, whereas the semi-finished product made by established 
technology does not have a stacked microstructure. 
Molybdenum oxide powder was treated with a potassium silicate solution in 
such a manner that, after reduction, a mixture of molybdenum with 0.175% 
by weight of potassium and 0.152% by weight of silicon was obtained. This 
treated molybdenum powder with a mean grain size of about 5 .mu.m was 
pressed into plates measuring 400 mm.times.170 mm.times.47 mm on a die 
press at a pressure of 3 MN. 
The plates were then sintered under an inert H.sub.2 gas cover, using a 
heating time of 3 hours and a hold time of 5 hours at 1700.degree. C. A 
portion of these plates was rolled according to established technology, 
beginning at a reshaping temperature of about 1400.degree. C., into sheets 
of 2 mm thickness in steps of approximately 10% reshaping at a time. 
During a final recrystallization annealing treatment at 1900.degree. C. 
during 15 minutes, no stacked microstructure was formed. The structure 
remained essentially fine-grained and had no longitudinal orientation. The 
creep rate of this sheet amounted to 
##EQU4## 
at 1800.degree. C. and a load of 10N/mm.sup.2. 
The remaining plates were rolled according to the process covered by the 
invention, beginning at a reshaping temperature of about 1400.degree. C., 
into sheets of 2.2 mm thickness in the same steps of approximately 10% 
reshaping at a time. 
After annealing under an inert H.sub.2 gas cover at 1100.degree. C. during 
5 hours, the sheet was rolled down in one step to the final 2 mm thickness 
at a temperature of about 700.degree. C. 
After a final recrystallization annealing treatment at 1900.degree. C. 
during 15 minutes, the sheet showed a good stacked microstructure. The 
creep rate of this sheet amounted to 
##EQU5## 
at 1800.degree. C. and a load of 10N/mm.sup.2