Okay, I know railroad spike knives are a pretty controversial topic among blacksmiths. Tempers flare and arguments ensue whenever folks ask if a railroad spike is proper steel to hold a cutting edge. What it comes down to is that the steel in the spikes don't have the right carbon content to allow them to hold a decent edge when forged into a knife. Made to flex and not crack under strain, spikes do their job of holding down track admirably. In order to be flexible, they are made of a medium carbon steel. Many out there claim that there are high carbon spikes, marked with an HC on the head. In fact, the spike I use in this tutorial is one of them. Many believe that they are not suitable as knives. Fortunately, through forge welding, we can laminate a higher carbon steel into the spike, creating a blade that has the tough medium carbon steel on the flats and an edge of high carbon steel that will stay sharp. We can now indulge our cravings of making knives out of railroad spikes.
Here is the railroad spike I selected with the high carbon inset. This insert is a portion of worn out file with the teeth ground off. I performed this grinding to make sure the surface was clean and free of rust or grease. This will help facilitate the welding. I also ground it to the width of the spike, including the taper. This will help create a nice neat weld. Any portion of the high carbon insert that is exposed has a higher chance of burning up when reaching the welding temp for the spike, so the insert should be protected by the medium carbon steel that has a higher burning point
It is also important to grind the end of the insert that will we within the spike to a wedge. This will allow the spike to conform to the insert's shape easier without leaving a gap at the bottom of the split. The taper of the wedge should be gradual, this allows the spike to hug the insert without spitting it out when hammering the split closed. Speaking of the split, I cut down the length of the spike, far enough to accommodate the insert. It is oriented so that the edges of the insert will become the cutting edge and the spine of the blade. All of this prep work is very important. Taking care of these details now makes the welding process, which is already tricky, a little less stressful.
Into the fire goes the spike. The end with the split must be hot enough to open the split up to accept the insert. I use charcoal in my forge, which is a pretty clean fuel. Welding can be impeded by impurities that burn off from dirty coal or non-ferrous metals. Clinkers, which form when the impurities of the coal melt, off gas substances that get in the way of welding and they also cover the air flow. This keeps the fire from getting hot enough to reach the welding temp. Charcoal doesn't form clinkers, so I don't have to worry about this.
The split has been opened and fluxed. Notice how portions of the steel look wet. This is the melted flux, which in this case is borax. Some of the borax that contacted steel to cool to fully melt it clings to spike down by the vise. The flux will coat the steel and keep rust scale from forming, which would get in the way of the weld. It will also bind to what little rust is present and pull it out when the weld is hammered closed.
After heating the spike once more, the insert is placed in the split and the split is hammered closed. Heat the whole thing once more and flux it well. At this point the spike is ready for the weld. Slowly heat the spike so that the welding temperature soaks into the core of the spike. I place the spike with the edge of the insert facing down so the heat can flow up through the split. I rotate the spike 180 degrees periodically so that it heats evenly. You will know when you have reached the welding temp when the flux begins to sizzle and the portion of the spike to be welded is glowing bright yellow/white. Do not heat the spike to the point of sparking. Sparking is a sign that the carbon in the steel is burning off, defeating the purpose of adding high carbon material. The welding temp is just below this point of sparking, so check the spike frequently enough to avoid over heating.
When at welding heat, remove the spike quickly from the fire and hammer the area to be welded, sticking the split together with the insert sandwiched in between. I did three rounds of this to assure the weld stuck down the entire length of the split. To the right is a picture of the edge after the final weld. Notice there are no gaps along the edge, just a solid piece of steel. Many times, a black line will be visible where a weld didn't take. This line becomes very obvious when that edge is ground shiny. Below is a picture of the tip of the knife along it's back edge. Ground
down past the black crust of oxidation to the shiny steel, you see that
there are no black lines.
If there are black lines you can attempt to flux and weld the spot again. It is definitely easier to do it right the first time, however. Another place you would see black lines, if the weld did not take, is along the flat of the blade. When you grind the bevel, this exposes the junction between spike and insert. In the picture below, you can see that the flat has no black lines, meaning the weld took. Do not be hard on yourself if you end up with spots where the weld does not take, it takes
a lot of practice to get the heat just right and the coordination to quickly close the weld before the steel cools to much. The knife, which was once a railroad spike, now has an edge of high carbon steel, that will hold a sharp edge. By hammering the blade evenly on both sides, the high carbon insert stays centered so that it becomes the cutting edge. After shaping and grinding, I quenched the knife in oil and tempered it to a light yellow. Sharpened to a razor edge, the knife now awaits a leather wrap for the grip and it's first task as an edged tool.
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