The carbon content of A36 is dependent on the thickness of the material. Thinner materials require less alloy addition than thick sections due to the increased mechanical work, i.e., rolling, required to produce thinner materials.

The tensile strength of steels is a function of the alloy composition and the manufacturing process. The manufacturer has to meet certain targets included in the material specification. There are different means of hitting those targets. A typical ASTM specification will list the mechanical properties and the ranges of chemistry. The manufacturer has the latitude of rolling the steel at different temperatures, passing the material through the rolls "x" number of times, and balancing the chemistry to produce the desired mechanical properties.

The latitude given the manufacturer for ASTM A36 is greater than for a material such as ASTM A992, which includes not only the required mechanical properties, but also the required carbon equivalency as a means placing greater control on the manufacturing process and ensuring good weldability.

The engineer that only considers the carbon content doesn't understand metallurgy. Like CWIs, doctors, and welders, not all engineers graduated in the top 10% of their class. I still remember the story my father told me about an engineer that got tired of waiting for an operator and decided he could drive a bulldozer down a flight of stairs. Another story for another day. Another consideration; not all engineers have a good background in materials science. Engineering encompasses many areas of specialization; there are mechanical engineers, welding engineers, chemical engineers, electrical engineers, ceramic engineers, aerospace engineers, highway engineers, I think you see my drift, all may include the job responsibility of designing welds. Like some welders, they may be hesitant to admit there is something they don't know and they may believe their position is in jeopardy if they seek outside advice. Silly welder, silly engineer.

In your case, there is no advantage that I see in using a filler metal that over matches the mechanical properties of the base metal. A filler metal with a tensile strength of 60 ksi or even 70 ksi should be adequate for the application that involves ASTM A36 base metal. The 4130 contains carbon, molybdenum and chrome in sufficient quantities that the deposit can be very hard, very strong, and, yes, very brittle if the cooling rate isn't controlled. If your engineer used the carbon equivalency formula found in AWS D1.1 Annex XI, he would discover the Ceq would be very high indicating potential problems with weldability.  

Good luck - Al

Well, your man needs to research how things are to be welded and why.  Depending on the application you don't just go and say use 4130 on A36 because it allmost has the same carbon content.  There are other goodies in that filler that might not be good with the other goodies in the A36.   In raceing they don't even use 4130 chrome-moly filler on chrome-moly tube chassis becuase it's to rigid and has no ductility. 
   So what are you going to do about this little problem you have at work or can you do anything?