The Art of Forging» Mizuno is unquestionably the world's leading iron forger. But what is that reputation built on?
What are the fundamental differences between a forged iron and a cast iron?
You can think of casting like making ice in your freezer - you are turning a liquid into a solid by using a mould. Cast irons tend to be made from a stainless steel alloy; the steel goes into a furnace, melts, and is then poured into a ceramic mould. When everything has cooled down the mould is broken, leaving the club inside. With forging, the club is fashioned from one piece of metal. At Mizuno we make irons out of bars of mild carbon steel; the bars are 10in long and about one inch in diameter. The metal is heated until it is red hot, then hammered and crafted into shape.
Single forged head at primary stage
Mass production cast heads
Why does Mizuno choose to forge its MP irons?
The forging process produces greater consistency and quality in the metal. Casting may be a more economical way to produce clubs - a cast head costs about half as much to produce as a forged head. But the downside of casting is that when the metal is poured into the mould, it always traps tiny bubbles inside the metal structure. We can go back to the freezer; no matter how carefully you pour the water, there are always bubbles in the ice. These bubbles make the face inconsistent; two shots from almost the same place can produce different results.
Can the average golfer feel those inconsistencies in a cast head?
Perhaps not if that's the only club you are hitting - but if you hit one and then hit one of our forged heads, which do not have any trapped bubbles, you will instantly tell it feels more solid and sweet. For the better player it is certainly obvious. We once tried to fool our in-house pro by giving him two identical-looking clubs to try. We told him both were made from forged carbon steel, but in fact one was cast stainless. The instant he hit it he turned to us and said: "What is that? It feels dead."
Why does a forged head sound so different to a cast head?
Because the air bubbles produced in the casting process affect sound vibration. You can think of it this way: fill two wine glasses with water, one with fizzy carbonated and the other with still. Then flick or tap the glass with a pen, or something. The sound is totally different. The fizzy water glass gives a dead 'pip' while the still water creates a longer 'di-i-i-ng'. The bubbles absorb the sound. And so it is with golf clubs. Cast heads dampen the sound quickly because there is air inside the metal. Our mild carbon steel forged heads, with no bubbles, produce a longer sound duration which gives more feedback.
Air bubbles within cast head
Some of the world's most popular wedges, including Titleist Vokey and Cleveland, are cast. What do you believe are the performance benefits of Mizuno's forged wedges?
Of course for short shots you need feel and distance control - and our forging process enhances both. The mild carbon steel we use for forging is a softer metal, giving the golfer extra touch; the increased vibration we have just talked about, created by the more solid head, is again going to give the golfer more feel, which gives him greater information and feedback on the shot. But also the purity and integrity of the metal produced by forging allows the player to strike the ball consistent distances, which boosts confidence.
Why do cast clubs tend to be made from stainless steel while forged clubs are made from mild carbon steel?
Mild carbon steel could be used for casting, but when it is melted its viscosity is high; it doesn't flow into the mould as well, and so does not fill up the detailed part of the mould as efficiently. Molten stainless steel is more liquid, so you can avoid bumps or not filling a recess. But also 17-4 stainless steel, the famous one, is very strong, twice as strong as mild carbon steel that is used for forged heads. The strength is good for design flexibility - you can make thinner walls in cavity backs for example - but its hardness means it cannot be forged so well; the material cracks or breaks when it is hammered.
You use 1025 mild carbon steel in your irons and wedges. Why?
Because it gives the perfect blend of hardness and feel. The '1025' means there is 0.25% carbon in the steel. The carbon content affects the hardness of the metal. If you go to more than 1% it gets hard, but also brittle; if you go lower than 0.1% the metal is too soft; it could deform while you play. We wanted a substance that was resilient but had feel, and which was ductile enough to stand loft and lie adjustment. And the 1025 steel was perfect.
Now you have found a new material, 1025E Pure Select mild carbon steel. What is the difference?
1025E Pure Select is a purer metal which will give even greater consistency. We believed the original 1025 steel was a wonderful metal for a golf club, but our quest for perfection led us to look for ways to improve it by reducing unnecessary elements that occur during forging - specifically phosphorous and sulphur. On rare occasions, these can slightly affect hardness in the face. Now we've found a way to do that. The new metal is also better at minimising so-called craftsman's marks that can be caused by bending the hosel for loft and lie alterations.
'Grain flow' is a Mizuno patent and trade mark(TM) What does it mean, and what are its benefits?
I mentioned that our irons are forged out of metal rods, or bars. These have a natural flow of the fibre, a grain. This grain acts as a strengthener/reinforcer, improving the block's integrity, consistency and durability. Think of it like concrete, reinforced by steel rods inside; the grain acts like the steel rods. Mizuno is the only company to arrange and control this flow to pass on a performance benefit to the golfer. We do our best to maintain this flow into the finished head. It makes the club stronger and more consistent. We achieve it by taking the bar and stretching about half of it into a narrower diameter. This end eventually becomes the hosel. But because it is a squeezing, stretching action, we maintain the flow of the grain that is trapped inside the bar. After this we put the half with the squeezed metal into the primary forging mould and then we forge it, so this hosel portion becomes the hosel and other part becomes the face - and we have minimal waste. Other companies don't do this - they do not care about the metal's grain. Rather than stretch the rod they pound it, hammer it. It creates lots of flash - unnecessary parts - which must then be taken out. That's usually done by milling and grinding. The whole process cuts through the natural flow of the fibre, which makes their irons weaker and less consistent.
Mizuno forging with tight grain lines
Competitor forging with non uniform grain
The Grain Flow Forging process gives your irons a one-piece construction. What are the benefits of that compared to other, welded-neck clubs?
It combines material consistency and strength with a very satisfying solid feel. Our competitors haven't the same experience in the forging of golf equipment. They've been forced to supply forged equipment as their tour players in particular will rarely accept a cast head. The forging houses they've used have tended to resort to forging their irons from two separate parts - a steel face welded to a steel hosel. Since our competitors re-introduced forged irons we started buying clubs to cut apart and check. We'll usually find the face and hosel have been welded the together. This weld breaks the flow lines of the metal and creates an inconsistency in structure from iron to iron. Although forged they cannot compete with Mizuno's Grain Flow Forgings on feel and consistency.
Competitor forging showing welded hosel
Is it true that Forged irons rust?
Not completely – it still depends on the material used. But yes, the 1025 mild carbon steel Mizuno uses will rust if it comes into contact with moisture. The chrome plating offers protection in most cases, but moisture can still penetrate surface abrasions made by sand and grit.
As long as you keep the clubs dry, with headcovers off between rounds, rusting won’t be a problem. Adding a touch of lubricant like standard WD40 occasionally will help too.
The rust issue is a trade off for the performance of a Grain Flow Forged 1025 mild carbon steel clubhead. You have to decide if the extra care it takes to keep the irons in good condition is worth the trade off.
Finally, how has Mizuno reached the forefront of forging technology?
For one thing our clubs are forged in the Chuo forging house - an exceptional facility that Mizuno has worked with exclusively for 38 years. But also we are always trying to improve: our new 1025E Pure Select steel reflects this. Today we believe we are the champions in the forging industry, but even now we put lots of resource - plenty engineers and lots of hours - to improve our process. That's our general direction. We believe that in the field of business, quality is the key thing. And we do not want our competitors to catch up with us.
Masao Nagai (55) is Mizuno Global Director of Golf R&D and has been based in Mizuno Golf's Research and Development department for 31 years. A trained metallurgical engineer, Masao designed the world's first mass-produced titanium driver - the Mizuno Pro ti-110 was released in 1990. It's head size was just 200cc. "But believe me," he says, "It was impressive at the time."