This is probably A. E. van Vogt's "Juggernaut" (1944), which was previously asked about and answered here. Though that answer was not accepted, it is almost certainly correct.
The story concerns the mysterious appearance of a bar of what looks like plain steel, which is collected in a wartime scrap metal drive. Metal from the batch including that bar can be shaped as normal steel, but, once fully "set" (which takes about two weeks), it cannot be remelted and has supernatural hardness (i.e. can't be bent at all):
"As you all know, obtaining information from a metallurgist"—he paused
and grinned inoffensively at Nadderly, whom he had invited down—"is
like obtaining blood from a turnip. Mr. Nadderly embodies in his
character and his science all the caution of a Scotchman who realizes
that it’s time he set up the drinks for everybody, but who is waiting
for some of the gang to depart.
"I might as well warn you, gentlemen,
that he is fully aware that any statement he has made on this metal
might be used against him. One of his objections is that thirty days
is a very brief period in the life of an alloy. There is an aluminum
alloy, for instance, that requires forty days to age-harden.
"Mr. Nadderly wishes that stressed because the original hard alloy, which
seems to have been a bar of about two inches square by a foot long,
has in fifteen days imparted its hardness to the rest of the bar, of
which it is a part.
"Gentlemen"—he looked earnestly over the
faces—"the hardness of this metal cannot be stated or estimated. It is
not just so many times harder than chromium or molybdenum steel. It is
hard beyond all calculation.
"Once hardened, it cannot be machined,
not even by tools made of itself. It won’t grind. Diamonds do not even
scratch it. Cannon shells neither dent it nor scratch it. Chemicals
have no effect. No heat we have been able to inflict on it has any
softening effect.
"Two pieces welded together—other metal attaches to
it readily—impart the hardness to the welding. Apparently, any metal,
once hardened by contact with the hard metal, will impart the hardness
to any metal with which it in turn comes into contact.
"The process is
cumulative and endless, though, as I have said, it seems to require
fifteen days. It is during this fortnight that the metal can be
worked.
The cause of the effect is discovered too late and the "infected" metal is too widely distributed to stop its spread. You remember correctly that the first person to realize this commits suicide:
It was two days after that that his mind, settling slowly to normalcy
from the excitement of the previous ten weeks, gave birth to a
thought. It was not a complete thought, not final. It was a doubt that
brought a tiny bead of perspiration out on his brow, and it prompted
him to sit down, a very shaken young man, and draw a diagrammatic
tree.
The tree began with a line that pointed at the word "Vulcan." It
branched out to "Factories," then to other factories. It branched
again, and again and again, and again and again and again.
It raced along railway tracks. It bridged the seas in ships and
planes. It moved along fences and into mines. It ceased to have a
beginning and an end. There was no end.
There was no color in Boothby’s face now. His eyes behind their owlish
spectacles had a glazed look. Like an old man, he swayed up finally
from his chair, and, hatless, wandered out into the afternoon. He
found his way home like a sick dog, and headed straight for his
workroom.
He wrote letters to Nadderly, to the chairman of the board of Vulcan,
and to the chief army and navy agent attached to the enormous steel
and iron works. He staggered to the nearest mailbox with the letters,
then returned to his work room, and headed straight for the drawer
where he kept his revolver.
The bullet splashed his brain out over the floor.
The government intentionally distributes the infected metal into Europe to stop the German war machine.
The story was originally published in the August 1944 edition of Astounding Science Fiction, and can be read online in its entirety courtesy of archive.org.
