Howdy back Chicken:
The 144 intake valves are the same material as all stock OEM valves used in 144, 170, 200 and 250 engines. They measure 1.467" across the face as compared to stock 200 exhaust valves at 1.38".
Some high performance, extreme duty exhaust valves are made from harder, mor durable materials and some even have sodium filled stems to help transfer heat. They are very expensive and must be custom build for our engines.
We used the 144 intakes as a larger exhaust valve upgrade when no other reasonable alternative was available. Both Dennis' 200 head and my 250 head, both with the 144 intakes on the exhaust side, have seen many miles of use and abuse with no problems (yet). We used used 144 valves which were available and cheap (If you have them).
I don't know of a source for hardened 144 intake valves.
Dennis has a set of Mike's (FSPP) SSI 1.5" exhaust valves ready for his next head. This is the way to go in terms of price, availability and quality.
The options and new products available for our engines has exploded since the advent of this Forum. What we had to do in the past is no longer the only option. Enjoy the bounty!
Adios, David