Experimental 6 inch (150 mm) Krupp armour plate from 1898.
The various types of iron and steel in armour are often a source of confusion. Without going into detail on the process of converting iron ore into iron or steel, the various types of metal used for armour can be briefly described. Iron with lower carbon levels is usually more malleable and flexible than iron with higher levels of carbon. The greater its carbon content, the stronger the iron is, but the greater its rigidity.
In the nineteenth century, armour was mostly made of cast iron, wrought, or rolled wrought iron, chilled cast iron, and steel later in the century. Cast iron is hard because it contains high levels of carbon and it is cast in a mould. Its carbon content is greater than in steel and therefore it is not malleable. Wrought iron is very low in carbon content making it comparatively soft compared to cast iron, but it is malleable. Rolled wrought iron is wrought iron that is broken up and reheated to create a higher quality of wrought iron. The American Civil War monitors and other ironclads were made of wrought iron. The first armoured ship was the French warship La Gloire, built in the 1850s to counter the development of the explosive shells that spelled an end to wooden-hull warships. Advancements in naval armament generally preceded similar developments for land fortifications. In the mid-1870s, the Italians began building steel ironclads, while land fortifications continued to use wrought-iron armour until almost the end of the 1880s.
In the late 1860s, steel armour was a hard enough surface to deflect shot, but steel plates were still too brittle to stand up to multiple hits. Thus in 1968, Grüson perfected its trademark chilled cast iron, a low-carbon cast iron. To produce this type of iron Grüson combined two types of pig iron: a highly carbonized type known as white iron, and a less carbonized soft grey iron. Layers of each type of iron, wrote Major A.G. Piorkowski in Scientific American, were ‘chilled by being cast in partly iron molds, thereby attaining an extraordinary hardness of surface, without apparently weakening the tenacity’. When the surface cooled, the two layers of white and grey merged so gradually that there was no marked line of separation in the metal. The outer surface held the carbon, which gave it hardness, while the interior layer was softer or more elastic. Thus, if an artillery round managed to break the surface, the area behind it, which was not brittle, did not shatter. In this manner, Grüson was able to combine hardness on the surface with tenacity in the metal below it to increase the resistance of the armour. He was able to cast his metal in any form and size required and to produce curved exterior surfaces, which was impossible with wrought iron. Due to their shape, his curved plates supported one another by remaining in position without bolts to hold them in place. Grüson’s process allowed the production of large plates that reduced the effects of a hit by distributing them over a large area. Other manufacturers who tried a similar process produced an armour with a distinct separation point between the white and the grey iron, which made the outer layer more vulnerable to shattering. In the 1880s, it was claimed that chilled cast iron had the ability to resist hits from the newly developed ordnance, including the Krupp armour-penetrating shells.
Grüson’s chilled iron amour was tested at the army’s artillery range at Tegel (just west of Berlin) between 1869 and 1871. During these tests, a 24-pounder (150mm) rifled gun, a 72-pounder (8.3in), and a 9.4in gun fired against embrasure plates and side plates. In 1871, a 210mm (8.3in) mortar fired at roof plates. During additional tests in 1873–1874, two types of 150mm (5.9in) guns with shell and solid shot fired against Schumann’s first chilled iron turret. The results of all these experiments were favourable. Another test against a second turret in the summer of 1874 revealed that additional, more rounded plates were needed. Grüson conducted additional trials, mainly at his firing range at Buckau outside of Magdeburg, between 1882 and 1885. Despite the success of this armour, a committee decided that heavier and thicker glacis plates were needed and future cupola plates should have a flatter profile curve. Unlike the glacis armour, it concluded, the turret armour had ‘considerable excess strength’.
In tests they conducted in 1886, the French determined that the high-explosive melinite shell of 1885 completely shattered cast-iron armour. As a result, they went back to using laminated or compound armour (see below), which they had abandoned in the late 1870s. In the 1890s, following the German lead, the French adopted steel armour produced with new casting methods. A major test at La Spezia, Italy in April 1886 involving a 100-ton Armstrong 16.9in gun came up with mixed results. However, it was limited to individual armoured plates. Shells that struck the sides of the Grüson chilled iron armoured turrets designed by Schumann shattered like glass. The cast off fragments inflicted little to no damage to the turret or other positions under the curved armour. When the Grüson armour took a hit, the result was usually a bright splash or a very slight indentation of a fraction of an inch.
Grüson chilled cast iron armour predominated on the Continent until the end of the century. It gradually replaced rolled wrought iron in fortifications and warships after 1875. Naval armour had gone from wrought-iron plates covering teak wood to compound armour consisting of a steel plate welded onto iron plates. This arrangement was not successful since the steel could shift or completely separate from the iron. In 1883, the Schneider Company tested all steel armour plates with some success. In the 1890s, the new ‘Harvey’ armour was developed. It consisted of soft steel with a carbonized surface to give it hardness and better resisting power.
Before the advent of Harvey armour, in 1875, the Italian navy held a competition at La Spezia to test new types of armour. The French Schneider Company dominated the competition with a new type of soft steel, which unfortunately broke under stress. A British manufacturer solved the problem of welding steel plates to iron in 1877. By the end of the 1880s, better quality steel armour replaced compound armour mainly for use on ships. In 1889, nickel-steel alloys improved the quality of armour plate. The next major improvement came in the 1890s when the American Hayward A. Harvey developed Harvey amour with hardened plate surfaces. This was done by covering the steel plate with charcoal and heating it at high temperatures for a few weeks then chilling it in oil and water baths successively. This process, which increased the carbon content on the surface and gradually decreased it inward, was very similar to Grüson’s, but greatly improved the qualities of the final product.
The American navy adopted nickel-steel for its ships to take advantage of its increased strength. Other nations followed suit. In the late 1890s, Krupp armour replaced Harvey armour for both naval and land fortifications. In 1893, Krupp developed a method similar to Harvey’s, but added chromium to the alloy to increase hardness. He also used carbon-bearing gases to heat the steel instead of covering the surface with coal, which yielded casehardened steel of greater strength than Harvey steel. The protection offered by 25.9cm (10.2in) of Krupp armour was the same as 30.4cm (12in) of Harvey armour. Krupp followed this up at the turn of the century with Krupp ‘cemented armour’ that included nickel, chromium, and manganese, which gave it greater elasticity and reduced spalling and cracking from direct hits. Krupp took over the Grüson Werks in 1893 and soon began producing steel armour for land fortifications.